150 384 LACHNELLULA WILLKOMMII (LARCH CANKER) Canker is a major disease of Larix decidua in Europe and is also found in Japan and North America. Infection biology Canker formation is typical of perennial cankers (386). Characteristic small circular orange fruiting bodies of the pathogen are produced on the surface of the cankers which develop on the branches and main stem. Recovery from canker occurs when the fungus fails to breach the periderm formed in the growing season. Losses 384–393 Canker diseases of trees: 384, dieback in Pinus Many Larix spp. (larches) are killed by L. willkommii. nigra plantation, northern Alps, caused by Sphaeropsis In a tree that survives, serious internal deformation can sapinea; 385, dieback of young P. nigra caused by render the whole tree useless for timber production. Gremmeniella abietina attack (photo copyright of Although seriously affecting only L. decidua in S. Murray); 386, canker caused by Lachnellula willkommii Europe, canker has been recorded on L. kaempferi and on Larix decidua; 387, dieback of Cupressus sempervirens L. sukaczevii. In general, L. kaempferi and hybrid caused by Seiridium cardinale; 388, canker on Castanea larch are quite resistant to L. willkommii. americana caused by Cryphonectria parasitica (photo copyright of G. Griffin); 389, basal canker on Eucalyptus Management tereticornis possibly caused by Cryphonectria cubensis Less-susceptible provenances, resistant species, and (photo copyright of E. Boa); 390, Nectria galligena regular hybrids may be planted; Sudetenland provenances of canker on cultivated apple; 391, leaf symptoms of L. decidua show greater resistance to canker than Apiognomonia veneta on Platanus x acerifolia; 392, crown alpine and lowland provenances. In North America, of Salix alba infected with Venturia saliciperda; 393, diffuse attempts are being made to eradicate larch canker. canker on Fraxinus excelsior caused by Pseudomonas syringae s.sp. savastanoi, pv. fraxini. SEIRIDIUM CARDINALE (CORYNEUM CANKER OF CYPRESS) Losses The disease causes serious dieback on C. sempervirens Cypress canker is the most important disease of in Mediterranean countries where cypress is a very Cupressus, causing problems wherever these trees important landscape tree. In East Africa, are grown for timber or for amenity. The sexual state C. macrocarpa and C. sempervirens were discarded of the pathogen (Leptosphaera) is known only from for timber production following a serious outbreak of California. Seiridium canker. Infection biology Variation in susceptibility occurs within and The pathogen enters through small wounds and between species in the family Cupressaceae. grows slowly through the bark, killing tissues; resin C. macrocarpa and C. sempervirens are particularly bleeding (resinosis) occurs from infected bark. susceptible to Seiridium. Chamaecyparis lawsoniana, Branches become girdled, with eventual death of C. nootkatensis, Cupressus arizonica, C. glabra, and distal parts (387), and the foliage fades to yellow and C. lusitanica are very resistant. finally to brown before falling. Continued growth of the tissue around the lesion leads to the formation of long, sunken cankers containing pin-head sized fruiting bodies (acervuli). The spores spread in water droplets and rain, but probably only over short distances.
MICROBIAL AND VIRAL PATHOGENS, AND PLANT PARASITES OF PLANTATION AND FOREST TREES 151 385 386 387 388 389 390 391 392 393
152 Table 6 Examples of important canker and other shoot diseases of trees Species Locality Principal hosts Damage Figures Comments Sphaeropsis Worldwide Pinus spp. Death of current 384 Common, but sapinea year’s shoots aggressive in warmer climates; can be severe Gremmeniella Northern Pinaceae Necrosis of bud 385 in 10–40-year-old abietina hemisphere base; death of plantations; occasional shoots; premature on seedlings needle loss; bark damage Very destructive disease; development Lachnellula Europe; rare in Larix spp., esp. Cankers on branches 386 associated with poor willkommii North America L. decidua and stems; girdling and choice of species for and Japan dieback; orange disc- site and climate, but shaped fruiting bodies, syndrome possibly Seiridium World-wide Cupressaceae 2–6 mm diameter more complex cardinale Dieback, host death 387 Significant disease of Cryphonectria European larch; most parasitica North America, Castanea spp. Stem cankers; 388 troublesome in southern dieback lowland and alpine Europe provenances Cryphonectria Tropical and Eucalyptus spp., Wound pathogen; 389 Most serious disease cubensis sub-tropical particularly large cankers at – of Cupressus spp. regions E. saligna, base of stem; Hypoxylon world-wide E. grandis, stem may 390 Chestnut blight; mammatum E. tereticornis be girdled 391 wiped out industries based on Castanea Nectria North America; Populus spp., Cankers on bark in North America galligena sporadic particularly of stems; dieback in Europe P. tremuloides, of branches; Serious threat to Apiognomonia P. tremula and premature death tropical plantations veneta their hybrids of Eucalyptus Europe, North Angiosperm trees, Regular cankers Most serious disease America, Chile of poplars in North including species on branches; America of Fagus, Fraxinus, dieback; red Very widespread pathogen; particular Malus, Pyrus, Acer, perithecia on problem in apple and pear orchards Populus canker margins Occasional severe Europe, Platanus spp., Dieback of twigs; attacks associated North America especially leaf lesions with moist spring P. occidentalis conditions. and P. x acerifolia P. orientalis fairly resistant
MICROBIAL AND VIRAL PATHOGENS, AND PLANT PARASITES OF PLANTATION AND FOREST TREES 153 Species Locality Principal hosts Damage Figures Comments ‘Beech Bark Europe, Fagus spp. Tarry spots on – Disease complex Disease’ bark; dimpling caused by infestation North America of bark; death of by felted beech vascular cambium; coccus, Cryptococcus large necrotic fagisuga, followed by areas on bark Nectria coccinea infection. Common in Cryptostroma Northern Acer Green stain in young plantations corticale Europe pseudoplatanus sapwood; wilting; dieback; black – Outbreaks appear in Venturia Europe, Salix fragilis, sporulation years following saliciperda North America S. alba, beneath dead bark warm, dry summers S. viminalis Necrotic black 392 Willow scab; blotches on leaves symptoms easily progressing onto confused with those shoots; withering of black canker of foliage; dieback (Glomerella miyabeana) Cryptodiaporthe Europe, eastern Populus spp., Bark necroses at bases of side – Frequent on P. nigra populnea North America particularly branches; dieback Italica P. nigra Shot-holes in – Bacterial canker leaves; depressed of cherry Pseudomonas Europe, Africa Prunus spp. lesions on branches mors- and stems with 393 Bacterial canker; most prunorum gum-like ooze; damaging disease of wilt; dieback ash in Europe. Similar Pseudomonas Europe Fraxinus; to pathogen causing syringae sub other Rough, irregular olive knot sp. savastanoi Oleaceae cankers, variable pv. fraxini in size, on bark – Fireblight of of branches Rosaceae, sub-family Erwinia Europe, Malus spp., and stems Pomoideae. Bacterial amylovora disease. Variation in North America, Pyrus spp., Rapid blackening susceptibility exists and wilting of between host New Zealand Sorbus spp. flowers and leaves; varieties and species death of very susceptible species – Bacterial canker of poplar. Great Xanthomonas Northern Populus spp. Irregular cankers variations in populi Europe and hybrids on branches and susceptibility are stems; dieback found between poplar clones
154 Management Losses and management The application of fungicides is impractical. In northern Europe, beech and ash are commonly Pruning out infected parts prolongs the life of the affected by this fungus but it is rarely necessary to tree, but in badly affected areas the replacement of control the disease outside the orchard. Cankered infected trees with resistant species is the only branches can be pruned off the tree. effective treatment. Selection and breeding programmes against S. cardinale are under way in PHYTOPHTHORA RAMORUM: SUDDEN Italy and Greece. OAK DEATH CRYPHONECTRIA PARASITICA (CHESTNUT BLIGHT) In the last 15 years, there has been increasing concern over the potential for Phytophthora species Following its introduction into North America to cause serious damage to forest ecosystems, as around 1900, on infected planting stock of Asian several newly discovered species of Phytophthora Castanea species, chestnut blight has been have been reported. Particularly noteworthy is the responsible for the demise of the American chestnut destruction caused to trees of the oak family in (Castanea dentata) for timber production. In the late California and Oregon due to infection by 1930s the disease was inadvertently introduced into P. ramorum (368, 369; Table 4). This pathogen is Italy and Yugoslavia, affecting the European sweet causing great concern, and is the subject of chestnut, C. sativa, and is spreading northwards into intensive quarantine measures throughout the southern France and Switzerland. world. It is present in Europe, where the pathogen appears to attack mainly hardy ornamental nursery Infection biology stock, such as species of Rhododendron and The spores are dispersed by rain splash and wind. Viburnum, although in the early years of the 21st Lesions, with abundant small pale-brown fruiting century, the pathogen has also been found on bodies, develop on the branches and main stems diseased species of Quercus (oak), Fagus (beech), (388). These eventually become girdled and die back. and Aesculus hippocastanum (horse chestnut) in The Netherlands and the UK. On ornamental Management shrubs, the disease is often restricted to leaf A decline in severity of the disease occurred in Italy blotching and dieback of affected shoots, and Yugoslavia when a mycovirus (‘d’ factor) whereas on many tree species, death of the crown infected the pathogen. Cankers caused by mycovirus- occurs following the development of cankering- infected strains of C. parasitica are less damaging. like symptoms on the secondary tissues of the stem or branches. In California, the problem The importation of Castanea planting stock and was first noted in the mid-1990s, when oak family barked timber into unaffected areas is now trees began dying in natural forests in coastal prohibited. counties. Lithocarpus densiflorus (tan bark oak) and related species were particularly badly affected. NECTRIA GALLIGENA (NECTRIA CANKER) Since these initial reports, P. ramorum has been found causing disease on a wide range of host Nectria galligena causes canker of apple and pear in woody plants, including species of Quercus, orchards and also affects other angiosperm trees. Umbellaria, Pieris, Vaccinium, and Camellia, and Regular, target cankers are formed on the branches on Sequoia sempervirens (coastal redwood). The and main stems of affected trees (390). potential for P. ramorum to cause serious damage to tree and shrub species on continents outside Infection biology North America has yet to be evaluated in full. The pathogen may enter bark through pruning The implications for disease management, if wounds, and swollen bark develops around infection P. ramorum did become more widespread, could sites. Small red perithecia of Nectria develop in prove enormous. clusters on the surface of these cankers.
MICROBIAL AND VIRAL PATHOGENS, AND PLANT PARASITES OF PLANTATION AND FOREST TREES 155 BACTERIAL CANKER OF POPLAR expand rapidly until large areas of the leaf surface are damaged. Sometimes spots are bordered by Bacterial canker is the most important disease discrete, dark zone lines but they may also expand affecting Populus spp. (poplar) in Europe, but it is unevenly over the needle or leaf. Some examples of absent in North America. The causal agent, foliage diseases are described below, but many other Xanthomonas populi s.sp. populi, is host specific, foliage diseases occur (Table 7). attacking only susceptible poplar species. Disease cycle and symptoms RED BAND NEEDLE BLIGHT OF PINE The bacterium is probably transmitted by rain (MYCOSPHAERELLA PINI) splash, on the prevailing wind, and by insects. Entry to host trees is probably via wounds, but Red band needle blight (Dothistroma blight) attacks bacteria may also gain entry via nectaries, may species of pine, but is a particular problem on hydathodes, and lenticels. It grows in the vascular Pinus radiata (Monterey pine, 394, 395) and cambium and a whitish slime fills the intercellular P. ponderosa (western yellow pine). It caused very spaces and exudes from small cracks on infected serious damage in East Africa, New Zealand, and branches in spring. Affected branches may be Australia in the 1950s. In East Africa, P. radiata is no girdled and die back but on older trees cankers can longer grown on a commercial scale because of the be present for many years. Here, large and diffuse disease. cankers develop due to the formation of callus around the lesion margin. Symptoms Dark red banding appears on the needles, leading to Losses caused their early abscission and consequent loss in tree Infection may lead to severe dieback; even small growth increment. Repeated severe attacks can lead numbers of cankers on a trunk render it useless to tree death. for timber. Management Management This disease provides one of the few examples of the The most effective way of managing canker is to economically viable use of fungicides in controlling plant resistant clones. Populus nigra and many a forest pathogen. Red band needle blight is treated P. x euamericana hybrids are completely resistant, by aerial spraying with fungicides in Australia and whereas some clones, e.g. ‘Brabantica’ and New Zealand. ‘Grandis’, are extremely susceptible to bacterial canker. The temperature-sensitive nature of the NEEDLE CASTS OF CONIFERS bacterium prevents serious disease development in warmer parts of Europe. Legislation in some Defoliation of conifers caused by needle cast diseases European countries requires the removal of diseased results in growth reduction and, in extreme trees as the source of inoculum. cases, death. Many fungi are associated with defoliation, including: Lophodermium seditiosum, DISEASES OF FOLIAGE L. sulcigena, Mycosphaerella (Dothistroma) pini, and Cyclaneusma minus on Pinus spp. (pine); Although ageing and senescent foliage of both Lophodermium piceae and Rhizosphaera kalkhoffii gymnosperm and angiosperm trees may be on Picea spp. (spruce); Rhabdocline pseudotsugae attacked by opportunistic pathogens, the critical on Pseudotsuga menziesii (Douglas fir); and damage occurs when young foliage is seriously Mycosphaerella laricina and Meria laricis on Larix attacked during rapid growth periods (Table 7, spp. (larch). Phaeocryptopus gauemannii causes Swiss 394–402). needle cast of Douglas fir, considered a particular problem in Christmas tree plantings, but more Many diseases of foliage are characterized by recently raising concerns in Douglas fir-dominated spotting. These spots may be restricted in size or may forests in the Pacific North West.
156 LOPHODERMIUM NEEDLE CAST OF PINES Management Where necessary, powdery mildews are controlled A range of Lophodermium species occurs on foliage using fungicide sprays. of two-needle pines. L. seditiosum causes serious needle cast of Pinus sylvestris (Scots pine) in TAPHRINA DEFORMANS (PEACH LEAF CURL) nurseries (396) and in plantations. It is also common on older trees, where damage appears insignificant, Leaf curl (402) is a common problem on Prunus although effects on growth have not been quantified. persica (peach), and closely related species, in most However, older trees act as disease reservoirs for areas where these trees are grown. Taphrina nearby nurseries. The symptoms of L. seditiosum are deformans infects leaves in early spring, causing easily confused with those of two other growth abnormalities. These ‘blisters’ usually turn Lophodermium species of little importance on living pale pink to bright red and the affected leaves abscise needles. L. pinastri occurs on abscised needles, in early summer. The pathogen exists as a free-living whereas L. conigenum proliferates on needles organism on leaves and shoots in the summer attached to fallen branches. months, and as spores during winter. Losses Taphrina is controlled in orchards with fungicides, The heaviest losses of pine seedlings occur in spraying just before bud burst and at nurseries adjacent to pine plantations, which are the 14-day intervals thereafter. The infected leaves main sources of fungal inoculum. should be gathered and burned. Management L. seditiosum can be controlled in nurseries using fungicide sprays. As the spores are viable only over short distances, infection can be avoided completely if nurseries are sited 2–3 km away from mature pines. POWDERY MILDEWS Mildews do not often have a wide host range but are usually host specific, attacking a limited number of species within a genus. The symptoms result from the white or pale-grey growth (largely superficial and often with a powdery texture) forming on the surfaces of leaves and young shoots. The pathogen withdraws nutrients from the host via haustoria which develop inside the host cell wall, invaginating the host cell plasmalemma. A significant powdery mildew in European forests is that caused by Microsphaera alphitoides on Quercus spp. (oak, 397, 398) and, rarely, Fagus sylvatica (beech) and Castanea sativa (sweet chestnut). Uncinula (Sawadea) bicornis and U. tulasnei attack Acer pseudoplatanus (sycamore, maple) and A. platanoides (Norway maple), respectively (399, 400), sometimes causing significant damage in amenity situations.
MICROBIAL AND VIRAL PATHOGENS, AND PLANT PARASITES OF PLANTATION AND FOREST TREES 157 Table 7 Examples of foliage diseases of trees Species Locality Principal hosts Damage Figures Comments Phaeocryptopus North America, Pseudotsuga spp. Pale-yellow foliage; – Particular problem premature defoliation where host planted gauemannii Europe, Australasia beginning with in unsuitable older needles; conditions. reduced increment Rhabdocline pseudotsugae agg. cause similar gross symptoms Mycosphaerella World-wide Pinus spp. Pale brown bands 394, Red band needle pini around needles, with 395 blight. Very serious dark red-brown disease in plant- margins; premature ations of Pinus defoliation; reduced radiata in East increment; death Africa, Chile and New Zealand Lophodermium Europe, Two- and three- Death of affected 396 Particular problem needle Pinus spp. foliage; elongated in nurseries and on seditiosum North America black fruiting young planting bodies (apothecia) stock; older trees on needles also attacked Microsphaera Europe Quercus spp. White powdery 397, Powdery mildew. alphitoides patches on upper 398 Affects plants leaf surface; of all ages affected leaves distorted; sexual stage (cleistothecia) form as tiny orange dots, turning black, in late summer Uncinula Europe Acer platanoides White powdery 399, Powdery mildew. tulasnei spots on upper 400 Uncinula bicornis leaf surface may attacks Acer coalesce to cover pseudoplatanus whole leaf; in late and A. campestre; summer affects both sides cleistothecia form of leaves as tiny orange dots, turning black Pleuroceras Europe Acer Large brown 401 Giant leaf blotch of pseudoplatanus lesions on leaf, sycamore maple pseudoplatani often around point of attachment of petiole to lamina (Continued)
158 Table 7 Examples of foliage diseases of trees (Continued) Species Locality Principal hosts Damage Figures Comments Rhytisma Europe, Acer spp., Black shiny – Tar spot. Very acerina North America particularly A. lesions, 1–2 cm common disease pseudoplatanus diameter on leaves wherever susceptible Acer Drepanopeziza Europe, Populus nigra, Dense dark brown spp. grow punctiformis North America P. deltoides, spotting on leaf, hybrids between coalescing. – Marssonina leaf these species Premature spot. Weakens defoliation. badly affected Reduced growth, trees which then dieback are more susceptible to Taphrina Europe, Prunus persica, Distortion of leaf secondary deformans North America, P. amygdalus and lamina, red pathogens Australasia related spp. blisters, defoliation in 402 Peach leaf curl. late spring Very common in peach and almond growing areas 394 395 396
MICROBIAL AND VIRAL PATHOGENS, AND PLANT PARASITES OF PLANTATION AND FOREST TREES 159 397 398 399 400 401 402 394–402 Diseases of foliage: 394, severe Mycosphaerella pini infection in the lower crown of Pinus radiata in New Zealand; 395, close-up of P. radiata seedling showing red banding on the needles due to M. pini infection; 396, severe needle browning on P. sylvestris caused by Lophodermium seditiosum (photo copyright of S. Murray); 397, leaf symptoms on Quercus robur (oak) caused by the powdery mildew Microsphaera alphitoides; 398, cleistothecia of M. alphitoides with regularly branched appendages; 399, leaf symptoms of Uncinula tulasnei (maple mildew) on Acer platanoides (Norway maple); 400, cleistothecia of U. tulasnei, the appendages of which lack branches; 401, large leaf lesions caused by Pleuroceras pseudoplatani on Acer pseudoplatanus; 402, typical red blisters and growth distortions on leaves of Prunus amygdalus infected with Taphrina deformans.
160 DECAY functional sapwood. Many entirely saprotrophic fungi degrade woody debris on the forest floor, and Decay of the roots and stems of trees causes the greatest others attack processed timber. economic losses in forestry. Apart from direct timber losses, it causes tree instability and leads to potential Decay is caused by highly specialized fungi, safety problems. Decay should, however, be considered usually in the order Hymenomycetes, although a few in its true natural and essential ecological role: it releases Ascomycotina in the Xylariaceae also degrade lignin. nutrients that have been locked up in lignified tissues for Some selected decay-causing fungi are listed in many years. This process removes woody debris from Table 8, and are illustrated in 403–412. the forest ecosystem, increasing soil fertility and organic matter content. Decay holes and decaying wood also Decay can be divided into brown and white rot, provide important habitats for other organisms. according to the type of decomposition. Brown rot fungi produce cellulases, polygalacturonases Much decay results from the growth of fungi (pectinases), and xylanases, degrading carbohydrate which are restricted to the heartwood during the polymers in cell walls; lignin is not decomposed. In lifetime of the tree, while few fungi grow within advanced brown rots, the wood has a dry and Table 8 Examples of important decay-causing fungi of the order hymenomycetes, and their niches. Several other decay-causing species are included in Table 5 Species Locality Principal hosts Damage Figures Comments Stereum spp. Ubiquitous Gymnosperm and Wound decay; white – Range of species, angiosperm trees rot, cankers; small including those in bracket-like or Amylostereum and resupinate fruit Haematostereum bodies lack pores` Chondrostereum Worldwide Wide host White rot; dieback; – Aggressive decay purpureum range, mostly host death pathogen, cause angiosperm of silver leaf trees; disease; particular occasional on problem on trees gymnosperms in the Rosaceae Ganoderma spp. Ubiquitous Very wide host White rot in 405 Taxonomy is range in angiosperm trees; confused angiosperms, root rot of palms in including plantations Palmae Fomes Europe, Angiosperm White rot; 406 Tinder fungus; fomentarius North America, trees, characteristic extensive decay of Asia particularly layering of hoof- heartwood Fagus, Betula shaped fruit body, with dark grey upper surface Polyporus Europe, Asia, Ulmus, Acer, White rot; large fan- 407 Common in squamosus Australia, rare Juglans in North shaped fruit body Europe America with pale fawn upper surface
MICROBIAL AND VIRAL PATHOGENS, AND PLANT PARASITES OF PLANTATION AND FOREST TREES 161 Species Locality Principal hosts Damage Figures Comments Bjerkandera Europe, Angiosperm Wound decay – Common on adusta North America trees; also Fagus spp. colonizes stumps of gymnosperms Meripilus Europe, Fagus spp.; Decay of upper 408 Decay of root giganteus North America rarely other root system; plate leads to angiosperm large pale–dark instability trees brown fleshy, annual fruiting bodies at base of tree Phellinus pini Northern Wide host range White rot; perennial – Considered one of hemisphere on gymnosperm bracket-like fruit the most serious gymnosperm trees body dark brown heart rot fungi in forests on upper surface North America Pleurotus Europe, Angiosperm White rot is flaky; – Oyster fungus; ostreatus North America, trees, notably annual gilled extensive decay Africa, Asia, Fagus, Quercus, fruiting bodies may occur Australasia Aesculus, Populus Laetiporus Europe, Wide host range, Brown cubical rot; 409 Common on sulphureus North America including both large yellow, Quercus spp.; also angiosperm and annual fleshy found on Taxus gymnosperm fruiting bodies trees Fomitopsis Europe, Angiosperm and Brown cubical rot; 410 Significant in pinicola North America gymnosperm bracket-like fruiting old-growth trees bodies are red- forests of western brown on upper North America; surface, with a common in yellow–pale forests of brown pore layer continental Europe Sparassis Europe, Pinaceae Brown cubical rot; 411 Decayed stems crispa North America, large, fleshy annual may snap in high Japan fruit bodies, winds cauliflower shaped Phaeolus Europe, Pinaceae; Brown cubical rot; 412 Decayed stems schweinitzii North America, occasional on large annual fruiting may snap in high Central America, angiosperm trees bodies produced winds; in North Japan, East Asia, on stem or near America trees are South Africa, stem base killed by New Zealand infections
162 404 403 405 403–412 Decay and fruiting bodies of decay-causing fungi: 406 403, brown rot in Picea abies – note the regular cubical cracking in the decayed wood (photo copyright of W. Bodles); 404, severe white rot in stem of Picea abies decayed by Heterobasidion annosum; 405, Ganoderma applanatum fruiting bodies (photo copyright of W. Mulenko, Marie- Curie-Sklodowska University, Lublin, Poland); 406, Fomes fomentarius on the stem of Betula pubescens; 407, Polyporus squamosus on Acer pseudoplatanus (photo copyright of S. Thompson); 408, Meripilus giganteus at base of F. sylvatica; 409, Laetiporus sulphureus on a fallen log of Quercus robur; 410, Fomitopsis pinicola on the stem of Picea abies (photo copyright of W. Mulenko); 411, fruiting body of Sparassis crispa at the base of Pinus sylvestris; 412, fruiting body of Phaeolus schweinitzii on wind-snapped Pseudotsuga menziesii. crumbly appearance, often with marked cracking, which may be of a cubical nature (403). Sheets of mycelium may develop in the cracks. White rot fungi, in addition to the enzymes described above, also produce several lignin- degrading enzymes (laccases, lignin peroxidases, and manganese-dependent lignin peroxidases). The peroxidases oxidatively cleave phenolic compounds from the lignified structure of the cell wall. White rot species differ in their relative abilities to degrade lignin and carbohydrates. Decay may be stringy, flaky, or pocketed (404) and is usually dry. In some
MICROBIAL AND VIRAL PATHOGENS, AND PLANT PARASITES OF PLANTATION AND FOREST TREES 163 407 408 409 410 411 412 fungal species, their ability to degrade early and late of the tree and, depending on conditions therein, wood differs: generally the thinner-walled early causes rot. A recent controversial hypothesis suggests wood is more readily degraded and results in a that some decay-causing species may exist laminated rot (379). endophytically inside the tree for years without causing any decay. Endophytic fungi can be isolated MODE OF ENTRY from sound wood where they appear to exist in stasis, but wounding changes the microenvironment Classically, decay-causing fungi were thought to enter within the tree, allowing air to enter and altering the trees through wounds. Damage to the bark of the tree gaseous balance. Such events then trigger growth of opens points where spores of decay fungi impact and the fungus and result in decay. germinate; the fungus then grows into the heartwood
164 Table 9 Examples of important rust diseases of trees Species Locality Principal hosts Damage Figures Comments Cronartium Europe 2-needle pines, Dieback of branches; 413 Alternate hosts: flaccidum Vincetoxicum, esp. Pinus sylvestris, death of host when Paeonia, Tropaeolum. P. pinea, P. pinaster main stem is girdled Microcyclic form: Endocronartium (Peridermium) pini Cronartium Europe; 5-needle pines Dieback of branches; 414–417 Alternate hosts: ribicola death of host when Ribes spp. North America main stem is girdled Cronartium South-east Pinus taeda, Kills young trees; – Alternate hosts: P. echinata, cankers on branches Quercus spp. (red f. sp. fusiforme USA, west P. elliottii and stems of older oaks); damage is trees lead to severe in quercuum to Texas breakage plantations Melampsora Worldwide, Salix spp. Premature 418, 419 Several species spp. where Salix defoliation; recognized. spp. are reduced increment, Alternate hosts: grown dieback Abies, Allium, Euonymus, Larix, Ribes. Affected trees often infected by secondary pathogens Melampsora Worldwide, Populus spp. Premature defoliation; 420 Several species spp. where Populus spp. are grown reduced increment, recognized. dieback Alternate hosts: Allium, Arum, Mercurialis, Pinus, Larix. Affected trees often infected by secondary pathogens MelampsoridiumEurope, Betula, Alnus Yellow spotting on 421 M. betulinum attacks leaves; reduced Betula spp.; spp. Asia, Japan, increment; severe M. hiratsukanum attacks lead to attacks Alnus spp. North America premature Alternate host: defoliation Larix spp.
MICROBIAL AND VIRAL PATHOGENS, AND PLANT PARASITES OF PLANTATION AND FOREST TREES 165 413 414 415 413–416 Rust diseases of gymnosperms: 413, top 416 dieback of Pinus sylvestris resulting from girdling by Endocronartium pini, the microcyclic form of Cronartium flaccidum; 414, abundant formation of aecidia of Cronartium ribicola on branches of Pinus strobus; 415, Ribes nigrum with a serious attack by C. ribicola; 416, yellow uredosori and red teleutosori of C. ribicola on underside of a Ribes nigrum leaf. RUST DISEASES (western white pine). These species showed good growth and high economic potential and were There are approximately 7,000 rust fungi which are planted in many countries, but it was soon realized obligate pathogens and require a suitable host plant for that they were highly susceptible to C. ribicola. growth. Some cause diseases of great economic importance to both angiosperm and gymnosperm trees At the end of the 19th century, C. ribicola was (Table 9, 413–421). Rusts have complex life cycles, with inadvertently introduced into eastern North America up to five different spore types (macrocyclic species, for on Pinus strobus stock imported from Europe. example Cronartium ribicola, 417), usually produced The disease was first recorded in Massachusetts in on two unrelated plants. In microcyclic species, the 1898 and spread to epidemic proportions in North number of spore types is reduced and often these rusts American forests, particularly where the climate are restricted to a single group of host plants. Two favoured basidiospore dispersal. The large natural significant rust diseases, white pine blister rust and forests of very susceptible American five-needle pine willow rust, are described in detail below but many species, and the presence of many native Ribes others attack woody plants. species in the forest understorey, contributed to the spread of the disease. CRONARTIUM RIBICOLA (WHITE PINE BLISTER RUST) C. ribicola is native to southeastern Europe, where it co-evolved with Pinus cembra and related five-needle pines, and the alternate host of Ribes. In the early 19th century, there were several introductions of North American five-needle pines into Europe, for example P. strobus (eastern white pine) and P. monticola
166 Symptoms Orange-coloured fruiting structures (aecidia, 414) The life cycle of this rust is illustrated in 417. Young arise annually in spring on the periphery of the canker trees and seedlings are very susceptible to infection, and produce large numbers of aecidiospores. The although obvious symptoms may not arise for three consequent dieback of branches and the tree is years. Needle infection occurs in spring, with red- associated with girdling lesions. brown flecks sometimes appearing on them, and quickly spreading into the shoots and branches. The aecidiospores from infected pines infect Ribes Perennial cankers form on branches and the main (415), where small yellow-orange uredosori (416) are stem, with pycnidia developing on canker margins. produced on the undersides of the leaves. Uredospores A nectar containing pycnidiospores forms in the (summer spores) can re-infect Ribes over long pycnidia. The nectar attracts insects which also visit distances. Later in the season, rust-coloured columnar other pycnidia, so exchanging pycnidiospores teleutosori produce teleutospores which overwinter on between opposite mating strains of the pathogen. fallen leaves, then germinate in early spring and release basidiospores to infect needles of susceptible pines. 417 B When cankers A girdle the main Pycnidia form on stem, top-dying affected shoots in occurs and summer whole tree may die Pathogen grows into shoots Aecidia form on cankers in Aecidiospores and branches of pine early summer, 2–4 years after infect Ribes in E infection summer DC Basidiospores infect needles Teleutospores germinate Teleutosori form on Ribes releasing basidiospores leaves in late summer Uredospores re-infect leaves of Ribes through summer months 417 Life cycle of Cronartium ribicola on five-needle Pinus and Ribes.
MICROBIAL AND VIRAL PATHOGENS, AND PLANT PARASITES OF PLANTATION AND FOREST TREES 167 418 419 420 421 418–421 Symptoms of rust diseases on angiosperm trees: 418, yellow uredosori and orange-red teleutosori of Melampsora epitea on leaves of Salix daphnoides; 419, dieback of this biomass willow following an attack by M. epitea; 420, rust spotting on underside of a leaf of Populus hybrid; 421, symptoms of Melampsoridium betulinum on leaves of Betula pubescens. Management most damaging is fusiforme rust of pines, caused During the 1930s attempts were made to eradicate by Cronartium quercuum forma specialis Ribes in North American forests. However, such control fusiforme. Pinus taeda and P. elliottii are is very difficult to achieve because of the persistence of particularly badly affected, although many other Ribes seed in soil. Fungicide spraying programmes in pines are attacked. The alternate hosts are Quercus the 1960s were also ineffective and unacceptable in the spp. (oaks); certain other members of the Fagaceae forest environment. are susceptible in artificial inoculations. Symptoms on pine include canker-like galls on the branches, A resistance breeding programme against where the aecidiospore stage is produced, C. ribicola was instigated in the USA in 1950. The ultimately resulting in girdling and dieback. Galls small numbers of white pine trees showing resistance may expand to reach the main stem and cause to the disease were selected for breeding. Good gains death of the tree. The disease became a major have been made, despite the development of races of problem in plantations of P. taeda and P. elliottii the pathogen able to overcome major gene resistance growing in areas with suitable conditions for in some host lines. Planting resistant Pinus strobus infection and pathogen development, namely the lines is now recommended in integrated management presence of suitable alternate hosts, frequent schemes in the USA. periods of high relative humidity (>97%), and temperatures of 15–27°C. OTHER SIGNIFICANT RUST DISEASES Other forms of C. quercuum cause gall rusts on OF GYMNOSPERMS various pine species in North America. There are many other rust diseases of significance on gymnosperms. In North America, one of the
168 WILLOW AND POPLAR RUSTS causes distortions in leaf and shoot growth, resulting in dieback (419). Later in the year, rust-coloured Salix spp. (willows) and Populus spp. (poplars) teleutosori form on the leaves. In the following planted for biomass production are often badly spring, the teleutospores germinate, releasing affected by rust diseases. As many as 20 species basidiospores to infect the alternate hosts on which of Melampsora, with alternate hosts including Larix, the pycnidial and aecidial stages of the life cycle occur. Allium, and Euonymus, attack willows worldwide. Susceptible willows and poplars are infected by M. epitea, the most damaging species in European aecidiospores in spring. biomass plantations, is divided into numerous sub- species. Poplars are also attacked by Melampsora Losses species, although it is difficult to separate many of Severe outbreaks of rust can rapidly defoliate sus- these on the basis of morphology alone. ceptible willows and poplars, causing reduced growth and sometimes death, with the particularly high risk Symptoms and life cycle of severe disease occurring in monoclonal biomass Yellow uredosori develop on leaves and soft shoots of plantings. Plants which are weakened by rust may affected plants in late spring, producing uredospores become more susceptible to attack by other diseases. that re-infect other susceptible hosts (418). Infection Table 10 Examples of wilt disease-causing pathogens of trees Species Locality Principal hosts Figures Comments Ceratocystis platani North east USA; Platanus spp., esp. 422, Canker stain of plane Ceratocystis fagacearum Mediterranean P. orientalis, P. x hispanica 423 trees countries 424, Nitidulid beetles act as North America Quercus spp., esp. red oaks 425 vectors; spread also facilitated by suckering Ophiostoma ulmi Europe Ulmus spp. – in red oaks Ophiostoma novo-ulmi North America; Ulmus spp. 427 Less serious than Europe – O. novo-ulmi infections; 30% of infected Verticillium albo-atrum Worldwide Very wide host range, die Verticillium dahliae especially Acer, Fraxinus, Tilia, Catalpa, Rhus Arguably the most significant disease of Erwinia salicis Northern Salix alba, S. caprea, 426 plants ever recorded Europe S. cinerea, S. fragilis, S. triandra Soil-borne pathogens. Cause problems in ornamental plantings but also present in forests Bacterial disease, watermark of cricket bat willow. Affected trees may die back. Reduces wood strength
MICROBIAL AND VIRAL PATHOGENS, AND PLANT PARASITES OF PLANTATION AND FOREST TREES 169 Management Symptoms The pathogens adapt rapidly to the presence of The young shoots and foliage of infected trees resistant host cultivars, therefore effective control over collapse rapidly, due to irreversible loss of turgor long periods is difficult to achieve. Fungicides are (425, 427), and the leaves later become yellow neither effective nor economic methods for control. (chlorotic). These symptoms, however, may also be The selection of more-resistant host genotypes and the caused by severe root disease, so that additional planting of clonal mixtures are the most promising symptoms are important in diagnosing vascular wilt management methods now under evaluation. diseases. The infection may be remote from the site of symptoms and marked disturbances occur in the WILT DISEASES water relations of the plant. The wilt is not entirely due to physical blockage of the xylem by the Wilt diseases caused by various pathogens occur pathogen, since toxins are produced which are throughout the world (Table 10, 422–427). The transported in the sap. Characteristic brown or green most important wilt of trees is Dutch elm disease; stain develops in the walls of the infected xylem indeed, this disease is arguably the most significant (423, 427). plant disease ever recorded. 422 423 424 425 426 422–426 Wilt diseases of trees: 422, mature Platanus orientalis dying as a result of infection with Ceratocystis platani (photo copyright of P. Tsopelas, NAGREF, Greece); 423, typical staining in trunk of P. orientalis, caused by infection with C. platani (photo copyright of P. Tsopelas, NAGREF, Greece); 424, oak wilt infection centre on Quercus ellipsoidalis – note the dead trees to the right, live trees to the left, and dying tree in the centre (photo copyright of J. Juzwik, USDA Forest Service); 425, close-up of attached leaves showing symptoms of oak wilt on Quercus ellipsoidalis (photo copyright of J. Juzwik, USDA Forest Service); 426, Salix alba var. coerulea showing dieback symptoms associated with Erwinia salicis (watermark disease).
170 OPHIOSTOMA NOVO-ULMI (DUTCH The beetles overwinter in breeding galleries in the ELM DISEASE) bark of dying or dead trees and the fungus develops saprotrophically in this bark. When adult beetles Dutch elm disease (DED) decimated Ulmus spp. emerge in spring, the sticky spores from coremia, (elm) populations in Europe, North America and formed by the fungus in the galleries, become western Asia in the 20th century. Outbreaks of a less- attached to the insects. Immediately after aggressive species, Ophiostoma ulmi, occurred in emergence, the beetles begin maturation feeding on Europe from 1918 to the mid-1960s, leading to the bark in the upper crowns of nearby elms. Spores deaths of approximately 30% of infected trees. In from the beetles’ bodies infect the feeding wounds in the UK, the incidence of the disease had fallen by the bark, germinate, and intrude into xylem vessels 1960, apparently due to reduced virulence of the of the elm. The pathogen rapidly spreads in the pathogen. From the early 1930s in North America, xylem sap and typical wilt symptoms arise in the however, a particularly severe outbreak of DED was elm. Tyloses form in the affected xylem vessels when in progress, with the susceptible Ulmus americana the plasmalemma of adjacent parenchyma cells suffering badly. expands into the vessel lumens. The bark of killed trees provides the breeding substrate for the next In the mid-1960s, a severe outbreak began in generation of beetles. southern England, and the strain of the pathogen responsible, now known as O. novo-ulmi, was Management shown to be identical to that found in North Control of the beetle vectors has been attempted America. The disease was imported on elm from this (by destroying infected wood, application of source and has since spread throughout almost all of contact or systemic insecticides, and the use of the UK. It is also present from southern Scandinavia wasps parasitic on Scolytus) but these measures to Mediterranean regions. have been ineffective. Injection of systemic fungicides, such as thiabendazole or the triazole The situation is further complicated by the fungicides propiconazole and tebuconizole, into presence of two highly aggressive strains of O. novo- elms can provide 30–40% protection, depending ulmi in Europe. The North American strain, on the size of the tree. Such injections are widely imported into western seaboard countries, is used for Dutch elm disease control (and control of spreading eastwards, while a second strain is other wilt diseases) in Europe and North America. spreading to the west from the Caucasus. In some Treatment is very expensive, however, and, as it is areas, the two co-occur and mate, giving the required every year, is worthwhile only on trees possibility of further highly virulent strains with high amenity value. emerging. Elm resistance breeding programmes in the USA Infection with O. novo-ulmi kills the aerial parts and The Netherlands attempt to exploit the natural of elms. However, in some trees their roots may resistance found in certain Asian species of Ulmus. sprout, hence many hedgerow elms still exist as large The most notable cultivar released to date is Sapporo shrubs or small trees. Their tops die back, however, Autumn Gold, an F1 hybrid of U. pumila x after several years of growth, once they reach a U. japonica. The apparent greater resistance of some suitable size for the beetle vector to feed on them. elms, for example U. glabra and U. laevis, to the Elms were large, significant components of the disease reflects the lower desirability of their bark to landscape, therefore marked changes in the amenity feeding beetles. value of affected areas has occurred. Life cycle In Europe the disease is transmitted from tree to tree (427) by several species of elm bark beetles (Scolytus scolytus, S. multistriatus, and S. laevis). The major factor in long-distance spread is the transport of infected logs within and between countries.
MICROBIAL AND VIRAL PATHOGENS, AND PLANT PARASITES OF PLANTATION AND FOREST TREES 171 Beetles emerge A 427 from galleries B Maturation with sticky coremiospores feeding on of the twigs enables pathogen coremiospores adhering to to enter host their bodies sap stream Characteristic brown streaking in infected sapwood G C Scolytus create Following breeding galleries infection, foliage in inner bark wilts and shrivels tissues Bark of dying trees is suitable ED for beetle breeding F Trees usually die within 12 months of infection Wilt shows Large numbers of as death of xylem vessels are foliage in blocked by tyloses crown 427 Life cycle of Dutch elm disease. VERTICILLIUM WILT Life cycle V. dahliae may survive for at least 10 years in soil as Many species of woody flowering plants, microsclerotia in dead and dying plant tissues; weed including forest trees, tree crops, and shrubs, are species also provide reservoirs of infection that can susceptible to wilt caused by species of maintain viability of the pathogen over many years. Verticillium. In most cases, the wilt is likely to be When roots grow, substances are exuded into the soil; caused by V. dahliae, although in some instances, these chemicals can stimulate germination of other species of the pathogen may be the causal V. dahliae microsclerotia, and the germinating hyphae agent. The disease is a particular problem on Olea are attracted towards the source of the exudates. On (olives), Acer spp. (maples), certain stone fruits contact with the surface of a fine root, the hyphae (Prunus spp.), and Pistachia vera (pistachio). penetrate the host, growing into the vascular cylinder Maples in amenity plantings can be very badly and entering the xylem. Production of specialized affected under conditions conducive to disease conidiospores (bud cells) in the xylem vessels enables development. the pathogen to spread systemically very rapidly in the host. Symptoms are generated as described above. Infection leads to the development of typical wilt symptoms, as described above.
172 OAK WILT suckering, large patches of trees may appear to die within woodlands. Death of Quercus spp. (oaks) through wilting was noted in the North Central States of the USA in the White oaks mid-19th century, although the causal agent was not When white oaks become infected with wilt, one or discovered until the mid-1940s. Oak wilt is caused a few branching systems may develop symptoms, by the fungus Ceratocystis fagacearum, and is most obviously in mid- to late summer. Dieback known to kill about 20 species of oak. Since the first varies greatly between trees, and leaf disease description of the disease, it has spread into Texas; it symptoms are rather similar to those found in is, however, confined to the eastern side of the Rocky infected red oaks. Mountains. Red oaks (sub-genus Erythrobalanus) are particularly susceptible and die within one to two Life cycle months of the first symptoms becoming apparent Human activity plays a major role in disease spread. (424, 425). In contrast, white oaks (sub-genus Beetles that transmit the pathogen are attracted Leucobalanus) may live for many years with the to damaged trees; any pruning activity or infection, taking up to seven years to die, although damage through building works can lead to infection. cases are known where the trees have remained alive Transmission of oak wilt by oak bark for at least 20 years. beetles – Pseudopityophthorus minutissimus and P. pruinosus – is similar to that of Dutch elm disease Symptoms vary between red and white oaks, and by Scolytus species. Beetles are attracted to recently with geographical region. A common symptom is dead oaks, and lay eggs. The larvae feed in the phloem the formation of grey to dark brown streaks in the tissues and pupate; the newly emerging adults may xylem vessels. Susceptible red oaks include Q. rubra pick up spores of C. fagacearum, if the disease killed (red oak), Q. palustris (pin oak), Q. tinctoria (black the trees. Maturation feeding of the newly emerged oak), Q. coccinea (scarlet oak), Q. imbricata adults in the crown of healthy trees then transmits the (shingle oak), and Q. nigra (blackjack oak). In the infection to new hosts. In the North Central States of Leucobalanus sub-genus, Q. alba (white oak), the USA, the pathogen is also vectored by a range of Q. bicolor (swamp white oak), Q. macrocarpa (bur oak sap beetle species, including Carpophilus, oak), Q. pinoides, and Q. obtusifolia (post oak) are Colopterus, Cryptarcha, Epurea, and Glischrochilus. damaged to varying degrees. These beetles are attracted to active spore mats, as described above, and pick up spores during feeding on Red oaks the sugary exudate. When visiting wounds, including In the North Central States, the first symptoms are pruning wounds on healthy trees, the infection is the development of necrotic patches on the leaves transmitted by contact. in summer, particularly at the leaf tip or on the edges. Leaves discolour, becoming a dull olive- On infection, the spores enter the vascular green, through red-brown to bronze (425). system of the tree and spread systemically. Wilting Premature abscission results; leaves on the ground is probably due both to vessel occlusion through can show all gradations of colour listed above, and tylosis formation, and toxin production by are often curled up at the edges. Dieback is rapid, C. fagacearum. Within eight weeks of defoliation, from the top downwards, and the whole tree can spore mats are formed under the bark on the main die quickly (424). Close examination of the main branches and trunk of the infected tree. As the trunk may show vertical cracks, covering the fungal material builds up, pressure is exerted on the developing spore mats, which may be 5–18 cm or dead bark, which cracks, allowing entry by sap more in length. Spore mats are grey–black felt-like beetles. structures that produce a fruity odour which attracts insects, birds, and squirrels; sometimes Management squirrels will gnaw through the bark over spore Several management protocols have been instigated mats to gain access to the sugary exudate. As many in the central states of the USA, where oak wilt is most red oaks commonly form clonal clumps through
MICROBIAL AND VIRAL PATHOGENS, AND PLANT PARASITES OF PLANTATION AND FOREST TREES 173 serious. As the disease attacks host trees in forests, Importance woodlands, parks, and gardens, there is an education Poplar mosaic carlavirus can cause 30% loss in policy that aims to inform the public of the symptoms increment in certain Populus (poplar) cultivars. Very and infection biology of the disease, plus likely methods little information is available in relation to losses to of management. Methods adopted include surveying virus infections in other trees, with most coming for the disease, management to reduce wounding or from work on fruit trees. to treat wounds with paint within minutes of damage occurring, root graft cutting to prevent Viruses are generally not host specific, infecting a physical transmission, and injection of the fungicide wide range of unrelated hosts. Forest trees, therefore, propiconazole (see management of Dutch elm disease). may become reservoirs of infection for other crops. For example, tobacco ringspot virus was found in VIRUS DISEASES Cupressus arizonica (Arizona cypress), tobacco necrosis virus has been isolated from Larix decidua Virus diseases of plants are extremely important (European larch), and strawberry latent ringspot was worldwide, causing huge economic losses. Most found in species of Aesculus. plants become infected by viruses, although some may have little apparent effect on plant growth. Virus diseases may appear unimportant in Some examples in trees (428–431) include poplar forestry, but this assumption probably reflects a lack mosaic carlavirus, apple mosaic ilaravirus, and of knowledge about tree viruses and their impact on cherry leaf roll nepovirus, while many other virus- forest ecosystems. like infections have been noted. 428 429 428–431 Symptoms of virus infections on trees 430 431 (photos copyright of I.E. Cooper, CEH, Oxford): 428, leaf of Populus showing symptoms of chlorosis caused by poplar mosaic carlavirus infection; 429, Betula leaves with ring spot symptoms resulting from apple mosaic ilaravirus infection; 430, walnut blackline, a graft incompatibility problem associated with cherry leaf roll nepovirus; 431, dwarfing of Betula sapling (left) inoculated with cherry leaf roll nepovirus. Control sapling is on the right.
174 Symptoms few groups are DNA-based. The RNA or DNA may Different symptoms may occur with the same type of be single- or double-stranded. Some plant viruses virus on different tree hosts. Generalized symptoms contain additional satellite strands of genomic are similar to many of those described earlier in this material, which may modify the infection process. chapter, and include chlorosis (sometimes known as Once inside the host plant, viruses replicate within ‘mosaics’, where mottling occurs over the leaf infected cells, producing large numbers of new virus lamina), yellowing of the veins, yellow circles or particles, which may then be transmitted to necroses on the leaves, and patches of killed bark. uninfected hosts by vectors. Increased (hyperplasia) or decreased (hypoplasia) cell division may also occur, or tree growth may Management cease (atrophy). Viral infections can cause Chemotherapy and/or thermotherapy, or meristem distortions of leaves or shoots, and damage to culture, are used to eliminate viruses from underlying woody tissues. horticultural crops. Control in trees, other than fruit trees, is generally not practical. However, genetic EXAMPLES OF VIRUSES IN FOREST TREES manipulation by inserting DNA encoding viral coat proteins or satellite RNA sequences into the plant Virus-like particles in gymnosperms were not genome has been tested against poplar mosaic recognized until the 1960s, and many of these agents carlavirus in Populus spp. grown for biomass have not been extensively characterized. In production. In any case, however, treated trees angiosperm trees, however, many virus diseases have would be exposed to re-infection once they are been reported (Cooper, 1993). Poplar mosaic planted out. These various control methods are carlavirus affects poplars (apparently the only hosts) therefore not suitable for forest trees, which have of the Populus deltoides group in Europe, North very long rotation times. America, and Japan. Foliar symptoms include spotting, mottling, and mosaic (428); leaves may curl PARASITIC PLANTS and develop small growths on petioles. Parasitism of plants by other plants is found in all Cherry leaf roll nepovirus occurs on several hosts, forested parts of the world, from the boreal regions including cherry (Prunus avium and P. padus), and to the tropics. The term ‘mistletoe’ encompasses over species of Betula (birch), Fagus (beech), Fraxinus 1,300 species of hemiparasitic flowering (ash), Juglans (walnut), and Ulmus (elm). plants which abstract water and minerals from a The symptoms are leaf rolling, diffuse mottling of host plant via specialized haustoria in the host’s leaves, chlorotic spots or lines, rings, and patches. vascular system, yet are mostly photosynthetic and Elm mosaic virus is common in shade elms in North therefore able to synthesize sugars. The most America, causing dieback of upper branches, sparse important mistletoes attacking forest trees are foliage, small leaves with chlorotic ring patterns, and Arceuthobium spp. (dwarf mistletoes, 432) and small overgrowths under the leaves in the interveinal various species of Viscum (433–435), Amyema regions. (436), Loranthus, Phoradendron, and Psittacanthus (leafy mistletoes). Mistletoes, however, have Life cycle important ecological roles in forest ecosystems, Mechanical transmission is probably very rare relying on animals for pollination and dispersal, and in nature, but occurs in normal horticultural practices forming an important habitat and food plant for via contaminated tools or during grafting. Otherwise, many forest organisms. In addition to those species to gain entry to host plants, viruses rely on vectors. with an obvious aerial parasitic habit, several species The most important vectors are insects (particularly of tree are hemiparasitic, relying on the roots of aphids), leaf hoppers, whiteflies, and mealy bugs. adjacent trees for sustenance. Species of Santalum Other arthropod vectors include mites. (sandalwood) are important examples of this habit (437); S. album wood has been used for centuries to Virus particles comprise an infectious nucleic acid produce fragrances. encapsulated within a protective protein coat. In most plant viruses the genome is RNA, although a
MICROBIAL AND VIRAL PATHOGENS, AND PLANT PARASITES OF PLANTATION AND FOREST TREES 175 432 433 434 435 436 437 432–437 Mistletoe infections of trees: 432, spring shoots of Arceuthobium oxycedri emerging from branch of Juniperus oxycedrus; 433, swelling of Populus host branch at point of infection by Viscum album; 434, multiple V. album infections in the crown of Salix alba; 435, multiple V. abietis infections in crown of Abies cephalonica; 436, Amyema (probably A. pendulum) in a roadside Eucalyptus sp., South Australia; 437, parasitic sandalwood (quandong; Santalum acuminatum), probably attached to adjacent Eucalyptus sp., South Australia (photo copyright of Mark Nurmela, Mount Barker, South Australia).
176 DWARF MISTLETOES amenity value, intensive control methods such as pruning, or fertilizing to increase host tree vigour, Forty-four species of Arceuthobium are found in the can be useful. northern hemisphere, mostly in North America, where they cause major losses on conifers. LEAFY MISTLETOES Identification is based on their morphology, cytology, chemistry, and host specificity Viscum album occurs in Europe, attacking a range of (Hawksworth and Wiens, 1996). Some of the most broadleaved trees, including Populus x euamericana significant species are: A. abietinum, which attacks (black poplars, 433), Malus domestica (apple), and species of Abies (fir) in western North America; A. species of Salix (willow, 434) and Tilia (lime). V. californicum, parasitic on species of Pinus (pine) in abietis occurs in Spain, France, Greece, and California and Oregon; A. douglasii, which attacks Romania; it primarily attacks Abies spp. (fir, 435) Pseudotsuga menziesii (Douglas fir) in western but may also affect species of Larix (larch) and Picea North America; and A. laricis, occurring on Larix (spruce). V. laxum occurs on Pinus spp. such as spp. (larch) in Oregon, Washington, and British P. nigra in southern Europe. Columbia. There is a single species recorded in the Old World, A. oxycedri, which is common on V. album and related species are dioecious, with Juniperus oxycedrus in Mediterranean regions and the female plants producing white berries containing in northern India (432). sticky seeds. Birds eat the berries and the seeds stick to their beaks; the birds vigorously rub their beaks Life cycle on tree branches to remove the seeds, which are then Seed is discharged explosively up to 10 m from deposited there. At germination, the radicle of the the mistletoe plant and sticks to needles on the mistletoe penetrates the bark of a host tree to reach surrounding trees. On wetting by rain, the the secondary phloem and sapwood, where it forms seeds germinate and penetrate the host tree. Mistletoes absorptive haustoria (sinkers). Mistletoe is typically flower from one to three years after infecting perennial, and its continued uptake of nutrients and a host. Male and female flowers are borne on separate water from the host has a weakening effect on the dwarf mistletoe plants, which produce new shoots tree. However, the loss in yield of affected trees can and flowers annually. The presence of witches’ be offset by the sale of mistletoe for Christmas brooms on a tree is a typical symptom of infection, decorations, and the deliberate cultivation of together with the formation of cankers, crown mistletoe on old apple trees is common in parts of dieback, and reduction in tree growth rate. northern Europe. Losses Several species of Amyema (family Loranthaceae) In North America, damage by dwarf mistletoes is are common on species of Eucalyptus, Acacia, second in importance to that caused by decay. Leptospermum, and Casuarina in Australasia, Approximately 5.5 million ha of the conifer forests including New Guinea. These leafy mistletoes have of California, Oregon, and Washington State are similar life cycles to V. album, but have distinctly affected by Arceuthobium. Losses in timber in the different adaptations to the varying hosts and USA have been estimated at 11.5 million m3 per environmental conditions found in Australasia. annum, with considerable additional impacts on A. pendulum (436), for example, is common on forest ecology and stand dynamics. eucalyptus trees in New South Wales and South Australia. Amyema and other leafy mistletoes are Management recognized as important nesting and feeding habitats Current methods aim to confine infection at an for Australian birds. acceptable level, rather than eliminating infections altogether. The use of non-susceptible conifer species and forest clearings as buffer zones is proving reasonably successful. In areas of high
MICROBIAL AND VIRAL PATHOGENS, AND PLANT PARASITES OF PLANTATION AND FOREST TREES 177 ACKNOWLEDGEMENTS Wiesław Mułenko (Marie-Curie-Sklodowska University, Lublin, Poland), Stan Murray (formerly The following colleagues kindly gave permission for of the University of Aberdeen), Stan Thompson the use of illustrations: Ellen Goheen and (Forres, Morayshire), Mark Nurmela (Mount Jenny Juzwik (USDA Forest Service), Eric Boa Barker, South Australia), Richard Robinson (CABI Bioscience), C.S. Hodges (University of (Conservation and Land Management Department, Gainesville), Don Barrett (formerly of the Oxford Western Australia), and Paolo Capretti (Universita Forestry Institute), Ian Cooper (CEH Oxford), Gary degli Studi, Firenze). Griffin (Virginia Polytechnic Institute and State University), W.J.A. Bodles (University of Aberdeen),
178 CHAPTER 9 Insect pests of some important forest trees Claire Ozanne INTRODUCTION a fungus or bacterium, into the tree during the feeding or egg laying process. Several insects Insects feeding on trees are described as pests when have complex symbiotic relationships with various they cause damage that is unacceptable to humans micro-organisms, which can cause disease when either in biological or economic terms. Across the inoculated by the insect into a susceptible world’s forests insects are estimated to take between host tree (438). 5 and 15% of tree foliage annually (Speight and Wainhouse, 1989), although some studies have 438 found insect herbivores to have a lower impact (0.7% in mature pine forest; Larsson and Tenow, 438 Bluestain fungus staining the wood of Picea sitchensis 1980), while others record much higher losses (up to (Sitka spruce). The fungus was transmitted by Trypodendron 20% in Australian rainforest; Lowman, 1984). sp. (Coleoptera: Scolytidae) and the tree was also attacked by Hylocoetus sp. (Coleoptera: Lymexylonidae). Most insect pests cause damage to trees when they feed, either as adults or juveniles, and may do so directly or indirectly. Direct damage results when part of the tree is removed – leaves, fruit, bark, wood, or root – and the material is usually consumed by the insect. Such feeding can reduce foliar or root area (and therefore productivity of the tree), may interrupt its nutrient and water flow, and can also cause changes to the form of the tree, such as bending of its shoots, branches, and even the trunk. Feeding on the seed and fruit reduces the viability of seed orchards and affects natural regeneration. Indirect damage results from the introduction of a disease-causing organism, such as
INSECT PESTS OF SOME IMPORTANT FOREST TREES 179 Insect pests can be divided into several categories Distribution based on the way in which they feed and therefore The aphid is widespread in the UK, with a the damage that they cause to the tree. These continuous distribution across northern France, categories include: suckers or sap feeders; bark and Denmark, Germany, Switzerland, and Austria. The shoot borers, and chewers (bark chewers and aphid also occurs in small areas of Scandinavia, defoliators). This chapter includes examples of these Latvia, and Iceland and has a limited distribution in main groups and covers a range of temperate and Tasmania, New Zealand, and the east and west tropical host tree species. coasts of North America (Day et al., 1998). SAP FEEDERS Impacts/ecology ELATOBIUM ABIETINUM WALKER The aphids feed on the underside of needles on the (HOMOPTERA: APHIDINAE, APHIDIDAE) – mesophyll cells; mature needles are preferred and the aphids are sensitive to variations in their terpene GREEN SPRUCE APHID composition. The first signs of aphid feeding are yellow patches on the needles (440), which then turn Description yellow and brown and may be lost during the summer The adult and juvenile aphids are green and (Koot, 1991). Outbreaks are recognized when the pear-shaped with dull red eyes (439) and distinct population reaches 0.5 aphids per needle in the tube-like structures or cornicles (thought to secrete summer population peak. The impact on height wax) on the abdomen. The juveniles or nymphs growth in Picea sitchensis (Sitka spruce) has been are wingless, and normally measure around 0.1–0.15 found to be a 10–30% reduction following severe cm in length. The adults may be winged or wingless, spring/early summer defoliation, and a narrowing in and are approximately 0.2 cm in length. The eggs stem diameter is evident both in the year following are oval and are yellowish-red to brown or black defoliation and even up to seven to eight years on. (Ministry of Forestry Canada, Day et al., 1998). Fertilizer and SO2 and NO2 treatments render trees more susceptible to attack, and the response Host trees to elevated CO2 is currently being investigated All species of Picea (spruce) that are currently (Day et al., 1998). in cultivation, with P. sitchensis, P. pungens, P. engelmanni, and P. alba all seriously damaged by aphid attack (Koot, 1991). 439 Apterous 439 440 (wingless) form of Elatobium abietinum (green spruce aphid) on Picea sitchensis (Sitka spruce) needle. 440 Damage caused by Elatobium abietinum to the foliage of Picea sitchensis – note yellowing of the needles.
180 Control cypress) and other species; Juniperus scopulorum, Epidemics are often reduced by weather changes J. virginiana, and J. sabinae (junipers); Thuja such as the onset of cool temperatures. Coccinellid occidentialis (white cedar) and T. plicata (western beetles and hemerobiids (brown lacewings), and red cedar); Chamaecyparis lawsoniana (Lawson other such predators are most likely to have a major cypress); Widdringtonia spp.; and Callitis calcarata impact on aphid densities. However, parasitoids (C. cupressivora). (parasites that develop within and kill the host – usually from the orders Hymenoptera, parasitoid Distribution wasps, or Diptera) also occur in aphid populations Cinara cupressi is found in Europe (Germany, (Day et al., 1998). Chemical control is not viable in The Netherlands, and the UK). It also occurs in plantations but may be used in nurseries. Sri Lanka, the USA (California, Pennsylvania, Arizona, Colorado, and Utah) and in Ontario, CINARA CUPRESSI BUCKTON (HOMOPTERA: Canada. Aphids also damage trees in British APHIDIDAE, LACHNINAE) – CYPRESS APHID Colombia, Israel, Poland, Slovakia, and Lithuania; Description but these attacks are likely to be caused by C. cupressivora. The latter species is also found in The adults and juveniles are brownish, soft-bodied Kenya, Rwanda, South Africa, Malawi, Mauritius, aphids, often covered in a grey waxy coating (441), Morocco, Zimbabwe, Syria, Jordan, Yemen, and the adults are approximately 2.4 mm long Burundi, Colombia, France, Greece, Italy, and (O’Neil, 1998). Cinara cupressi is a member of a Turkey. species complex which includes C. cupressivora, C. sabinae, C. cupressa, and an as yet unnamed Impacts/ecology species (Watson et al., 1999). Aphids pierce the bark of tree twigs and feed on the phloem sap, resulting in death of the twig and leading Host trees to yellowing of the foliage, especially in tropical dry The species complex feeds on trees in the seasons (442; O’Neil, 1998). Ants often tend the Cupressaceae (cypress family) (Watson et al., 1999). aphids and move them around the tree, so increasing These include: Cupressus macrocarpa (Monterey 441 442 442 Damage to Cupressus sp. (cypress) in Malawi caused by Cinara. 441 Cinara cupressi (cypress aphid) on the young shoots of Cupressus macrocarpa (Monterey cypress).
INSECT PESTS OF SOME IMPORTANT FOREST TREES 181 aphid spread. Sooty moulds commonly grow on the are similar to the adults, although smaller in size. honeydew exuded by the aphids, covering the leaves Eggs are laid between the new leaves on young host and reducing the photosynthetic capacity of the tree. shoot tips and the insect can complete its life cycle in If infestations are severe, feeding may cause tree 10–15 days when conditions are optimum. mortality. Loss of productivity from aphid infestations in East and Central Africa is estimated at US$13.5 Host trees million per annum (Wingfield and Day, 2002). Found only on Leucaena spp. (leucaenas). Control Distribution The parasitoid wasp Pauesia juniperorum could be The psyllid is native to Central America but the pest used as a control agent in the tropics as it will only began to spread towards the west in the early 1980s parasitize aphids in the sub-family Lachninae. and is now pantropical in distribution. In France and South Africa, Pauesia species introduced to control similar aphids began to control Impacts/ecology populations within two years, and studies by IIBC Both adults and nymphs feed on sap in young buds, (International Institute of Biological Control, shoots, leaves, and flowers of Leucaena trees. After unpublished data) have shown that in the laboratory, an attack, newly emerged leaves will not fully Pauesia juniperorum can cause substantial mortality unfurl, and shoots are subject to wilting; of Cinara cupressi (O’Neil, 1998). The aphid may also susceptible varieties may lose their leaves and, in be controlled by early applications of chemical agents severe cases, die (Geiger and Gutierrez, 2000). containing Pirimicarb. A sooty mould often grows on the honeydew produced by the psyllids, covering the leaves and HETEROPSYLLA CUBANA CRAWFORD reducing photosynthesis. In the Asia Pacific region (HEMIPTERA: PSYLLIDAE) – LEUCAENA PSYLLID the psyllid caused great damage within the first Description two years of arrival (Heydon and Affonso, 1991), with losses in potential productivity of up to 33% The adult psyllid (443) is yellow, winged, and (Oka, 1989). approximately 2 mm long (Hertel, 1998). Nymphs 443 Winged Control (alate) adults of 443 The incidence of the psyllid may be reduced by wide Heteropsylla cubana (leucaena spacing of newly planted trees to reduce humidity in psyllid) on young the stand. Chemical control has proven ineffective shoots of against the psyllid, although the use of dimethoate Leucaena. has been recommended in some areas. Biological control agents such as the parasitoids Psyllaephagus yaseeni Noyes (Encyrtidae) and Tamarixia leucaenae Boucek (Eulophidae), and the coccinellid beetle predator Curinus coeruleus, have had some efficacy in controlling populations in Hawaii and Indonesia (Nakahara et al., 1987; Mangoendihardjo and Wagiman, 1989; Wagiman et al., 1989). However, no highly effective agent has yet been found.
182 ADELGES TSUGAE ANNAND (HEMIPTERA: Impacts/ecology ADELGIDAE) – HEMLOCK WOOLLY ADELGID The adelgid feeds on the tissue at the base of the Description needles, causing nutrient loss and dehydration (McClure, 1996). This feeding, together with the The adults are small reddish-purple insects, injection of the sheath of saliva from around the approximately 1 mm in length, and are covered in a thread-like mouthparts of the adelgid, results in white waxy secretion (444). There are winged needle loss within a few months of infestation. (alate) and wingless (apterous) forms of adult; the Subsequently, the buds, twigs, and branches die and latter are slightly smaller in size. The nymphs are eventually the trees may be killed within three to similar to small wingless adults. The adults lay four years if the attack is left uncontrolled (445; around 50–100 orange-brown to black eggs in a Young et al., 1995; McClure, 2005). The insects are batch, and up to 300 in a lifetime. The eggs are dispersed by wind, birds, and mammals (McClure, covered in white wax. 1990), and populations can spread up to 25 km per year (Yorks et al., 1999). Host trees The primary hosts for the sexual phase of the insect’s Control life cycle are Picea trees (spruces) – this part of the The current control strategies include early detection life cycle has not yet been recorded to have been and the removal and destruction of infected trees. completed successfully in northern America, where The promotion of good silvicultural practices the insect has recently invaded. The secondary hosts designed to prevent tree stress may reduce the rate of in northern America are Tsuga canadensis (eastern attack. Trees can be sprayed with horticultural oils hemlock) and Tsuga caroliniana (ornamental and chemicals such as Imidacloprid; this treatment is Carolina hemlock). most effective in urban situations (McClure, 2005). Biological control agents such as the predatory Distribution coccinellid beetle Sasajiscymnus tsugae (Japanese The adelgid is native to Japan and China but ranges lady beetle; Cheah et al., 2004) and the oribatid mite in North America from Nova Scotia to Minnesota, Diapterobates humeralis have provided some and southwards to northern Alabama (Baumgras control in plantations. The efficacy of a predatory et al., 1999). It is currently found in approximately derodontid beetle, Laricobius nigrinus, is currently 25% of Tusga canadensis forests in North America being investigated (Lamb et al., 2005). (Zilahi-Balogh et al., 2002). 444 445 445 Stand of Tsuga sp. 444 White waxy deposits on the needles of Tsuga sp. (hemlock) heavily (hemlock) caused by secretions from the adults of defoliated by Adelges tsugae (hemlock woolly adelgid). (Photo Adelges tsugae copyright of Christopher R Webster, Michigan (hemlock woolly Technological University, USA.) adelgid). (Photo copyright of Christopher R Webster, Michigan Technological University, USA.)
INSECT PESTS OF SOME IMPORTANT FOREST TREES 183 BARK AND STEM BORERS (pines), and Pseudotsuga (Douglas fir and other DENDROCTONUS MICANS KUGELANN species). Picea abies (Norway spruce) is the main host (COLEOPTERA: SCOLYTIDAE) – GREAT in Europe, but infestations of P. orientalis (oriental EUROPEAN SPRUCE BARK BEETLE spruce) and P. sitchensis (Sitka spruce) also occur. In Description eastern Eurasia the main host tree is P. orientalis. This is a large black cylindrical beetle 6–9 mm in Distribution length, with a covering of orange-brown hairs D. micans is the principle Eurasian species in this (446). The larvae, which feed in aggregations, are genus. Its range stretches from northern Japan and typical in form for this family, and legless with Sakhalin Island across northern and western Europe, pale cream bodies and brown head capsules (447). The including Austria, Belgium, the Czech and Slovak life cycle of D. micans is not as predictable as other republics, Denmark, Finland, France, Germany, The related pest species of bark beetle, and adults exhibit Netherlands, Sweden, Romania, Turkey, the UK, the pre-emergence mating (King and Fielding, 1989). former USSR, and former Yugoslavia. Host trees Impacts/ecology The beetles will feed on a range of conifers, including D. micans breeds in living and healthy trees, and the Abies (firs), Larix (larches), Picea (spruces), Pinus larvae feed gregariously in order to overcome tree defences (King and Fielding, 1989). An attack is, 446 however, more common when trees have been damaged or are weakened in any way, and the degree 446 Adult of Dendroctonus micans (great European spruce of bark lignosuberization (impregnation with a bark beetle) – note its covering of orange-brown hairs. complex of carbohydrates and fatty acids) is (Photo copyright of Forestry Commission, UK.) known to affect susceptibility. The larvae excavate galleries under the bark, destroying the adjacent cambium; several attacks, perhaps over subsequent years, may result in ring barking the tree and causing death (Forestry Commission, 2002). Resin tubes and resin bleeding from tree stems characterize a larval attack (448). The fertilized females disperse by 447 Dendroctonus 447 448 Damage to 448 micans legless Larix sp. (larch) larvae feeding in caused by aggregation – D. micans – note note the contrast the resin bleed between their from its trunk. creamy-white bodies and brown heads. (Photo copyright of Forestry Commission, UK.)
184 flight (21–23°C threshold) or by walking to Impacts/ecology neighbouring trees, and epidemics are most This beetle is an aggressive pest (Byers, 1989). Many frequently associated with the leading edge of an beetles cooperate to attack single trees within a expanding population. stand, and these trees are frequently killed. Such cooperation is necessary to overcome the resin Control defences of the tree, and is achieved using an A survey of beetle distribution, sanitation felling aggregation chemical or pheromone (methyl butenol of infected trees, and the creation of quarantine and cis-verbenol), which is produced by feeding or protected zones from which timber cannot be males (Bakke and Strand, 1981; Byers, 1989). moved help to limit the spread of an infestation. Galleries are excavated in the thin phloem layer Biological control with the predatory beetle situated 2–4 mm under the host bark (449), and the Rhizophagus grandis (Gyllenhal Rhizophagidae) sapstain fungus Ceratocystis polonica is introduced is also highly effective. In the UK, plantation control (450; Webber and Eyre, 2003). The host tree is killed by R. grandis is predicted to reduce losses caused (451) by a combination of tissue damage by the by this bark beetle from 10% to 1% (Forestry beetles and blocking of the phloem by the fungus. Commission, 2003). IPS TYPOGRAPHUS LINNAEUS (COLEOPTERA: 449 SCOLYTIDAE) – EIGHT-SPINED SPRUCE BARK BEETLE (LARGER EUROPEAN SPRUCE BARK 449 De-barked trunk showing the galleries created by BEETLE OR SPRUCE ENGRAVER) Ips sp. (spruce bark beetle) larvae. Description 450 450 Cross-cut This is a moderate- to large-sized and cylindrical log showing brown beetle (4.2–5.5 mm), with four spines on evidence of the lateral edge of each wing case (elytron) in sapstain fungus males (eight in total). The anterior part of the transmitted to pronotum (plate covering the upper surface of the the host tree by first segment on the thorax) is rough and the body Ips sp. is fringed with yellow hairs (Whittle and Anderson, 1985). The larvae are white and legless, with a light- brown head capsule. They grow to about 5 mm in length. Host trees The main and preferred host is Picea abies (Norway spruce), but the beetle will also attack Larix (larches), Abies (firs), and Pinus (pines). Distribution It occurs in most of Continental Europe and northern Asia, including China, Japan, Korea, and Russia’s far east.
INSECT PESTS OF SOME IMPORTANT FOREST TREES 185 Control with partially hidden brown head capsules and Sanitation felling may be used to limit the spread of brown horns on the last abdominal segment this beetle, but chemical control is not considered to (McCullough and Katovich, 2004; Global Invasive be economically viable. Pheromone lures containing Species Database, 2005). the aggregation pheromones are used to monitor populations and to trap beetles. A wide range of Host trees natural enemies is currently being investigated In its native range the beetle can be found in Fraxinus for their effectiveness in control, such as the chinensis, F. rhynchophylla, F. mandshurica var. predatory clerid beetle Thanasimus formicarius, japonica (ashes), Ulmus davidiana var. japonica the hymenopteran parasitoids Dendrosoter (an elm), Juglans mandshurica var. sieboldiana middendorffi, Coeloides bostrichorum, Tomicobia (a walnut), and Pterocarya rhoifolia (a wingnut) seitneri, and Roptrocerus xylophagorum, and the (Haack et al., 2002; Nomura, 2002). In North America parasitoid wasp Rhopalicus tutela (Morrisey, 1996; it has been found attacking Fraxinus pennsylvanica Gregoire, 2003). (red-green ash), F. americana (white ash), and F. nigra (black ash), as well as several other ash species and AGRILUS PLANIPENNIS FAIRMAIRE cultivars (MacFarlane and Meyer, 2005). It is predicted (COLEOPTERA: BUPRESTIDAE) – that most ash varieties will be susceptible. EMERALD ASH BORER Distribution A. planipennis has a native range stretching from the Description far east of Russia across China, Korea, Taiwan, and The adult beetles are metallic green in colour with Japan. In 2002 it was discovered in North America an elongate, wedge-shaped body ranging in size and has now spread to Michigan, Ohio, Indiana, from 7.5 to 14 mm long and 3.0 to 3.4 mm wide Maryland, and Virginia, and to Ontario, Canada (452). The beetles have a ridge along the front (MacFarlane and Meyer, 2005). margin of the wing cases and normally have black, indented or kidney-shaped eyes. The mature larvae are long and thin (26–32 mm long), cream in colour, 451 Pinus sp. 451 452 (pine) trees attacked and killed by Ips sp. 452 Adult beetle of Agrilus planipennis (emerald ash borer). (Photo copyright of Andrew J Storer, Michigan Technological University, USA.)
186 Impacts/ecology currently limited to trunk spraying with fungal The adults feed on tree leaves but rarely cause spores of Beauvaria bassiana, which are reported to significant damage. The larvae burrow through the cause approximately 50% mortality in emerging bark and feed on the cambium layer and newly adults (Smitley and McCullough, 2004). forming phloem and xylem. This activity produces snaking tunnels, packed with frass (faecal pellets, PHORACANTHA SEMIPUNCTATA FABRICIUS 453). These galleries can extend for up to 50 cm and (COLEOPTERA: CERAMBYCIDAE) – interfere with the transport of water and nutrients; the bark splits, affected portions of the tree die and, EUCALYPTUS LONGHORN BORER after three to four years of infestation, a tree may be killed. Since ash species are widely distributed in Description North American forests, emerald ash borer is likely The adult beetle is 2.5–3 cm in length, and a shiny to substantially change forest diversity and dark brown to black in colour. A buff-coloured, dynamics. Significant economic effects are also zigzag patch crosses the elytra (wing cases), the predicted, as ash trees provide a range of important anterior portions are covered by small depressions wood products and are often grown in urban areas (punctate), and the elytral tips have a buff-coloured where infestations will need intensive management spot (454). As is characteristic of this beetle family, (MacFarlane and Meyer, 2005). the antennae are as long as or longer than the body (longer in the males). The larvae are cream-coloured Control with a retracted head capsule and effectively legless. Limiting spread of the beetle over long distances by They can grow to 3 cm in length (Bain, 1976). the implementation of quarantine zones and restricting the movement of trees and wood products Host trees can be effective. Chemical control may be carried out Many species of Eucalyptus, with E. diversicolor by topical application of trunk and foliage sprays (Karri), E. globulus (blue gum), E. nitens, E. saligna such as Cyfluthrin, but the timing is crucial, as it (Sydney blue gum), and E. viminalis (manna gum) must coincide with the emergence of the adults. particularly susceptible. Systemic chemical application using soil drenching (e.g. Imidacloprid) or injection (e.g. Bidrin) may be Distribution used to kill larvae already present in the tree (Smitley The beetle is native to Australia but is also found in and McCullough, 2004). Biological control is New Zealand, South America, South Africa and Zambia, the Middle East, the Mediterranean region, and in California, USA. 453 454 454 Adult of Phoracantha 453 Agrilus planipennis (emerald ash borer) larval galleries semipunctata under tree bark of Fraxinus sp. (ash). (Photo copyright of (eucalyptus longhorn Jessica Metzger, Michigan Technological University, USA.) borer) – note the buff-coloured patch on the wing case. (Photo copyright of C Fitzgerald, DPI Queensland, Australia.)
INSECT PESTS OF SOME IMPORTANT FOREST TREES 187 Impacts/ecology Control The adult beetles are attracted to recently dead and Cultural control, including the removal of cut logs dying trees or branches, where the females lay eggs and dead and dying material, can reduce the impact under loose bark or in crevices. Young larvae either of the pest. The choice of good tree species for tunnel straight into the inner bark, or may feed at the site is the key to ensuring that stress and therefore the surface, leaving a characteristic score mark, susceptibility are minimized. Parasitoids from the before moving under the bark. The attack is also beetle’s native range, such as the Hymenoptera characterized by sap bleeding. The damage results in Avetianella longoi and Syngaster lepidus, show wilting of foliage as translocation in the phloem is considerable promise in controlling populations decreased or ceases, branch die back, and sometimes across the current distribution (Hanks et al., 1995; mortality results (455). The larval galleries are Millar et al., 2002). For example, in California, 90% large (456) and may be up to 1 m long and 3 cm wide; of eggs in the field have been found to be parasitized so one gallery can ring bark and kill a tree (Hagen, by the introduced Avetianella longoi (Hanks et al., 1999). Trees are particularly susceptible when under 1995). Chemical control is regarded as unsuccessful drought stress, but under good site conditions the for this pest, although Bacillus thuringiensis (B.t.), beetle may not be a pest (Dreistadt et al., 1994). neem, and permethrin have been used on occasion. 455 ANOPLOPHORA GLABRIPENNIS MOTCHULSKY (COLEOPTERA: CERAMBYCIDAE: LAMIINAE: 455 Stand of Eucalyptus sp. killed by Phoracantha LAMIINI) – ASIAN LONGHORNED BEETLE semipunctata infestation in Queensland, Australia. Description The adults are large (20–35 mm long), shiny black beetles, with around 20 white dots on each wing case (457). The beetles bear the characteristic long antennae of the Cerambycidae (1.5–2.5 times body length); these antennae are striped with white bands. The legless larvae are cream in colour, up to 50 mm in length, and have a brown head capsule. 456 Galleries produced 456 457 by Phoracantha semipunctata larvae in Eucalyptus sp. in Queensland, Australia. 457 Adult beetle of Anoplophora glabripennis (Asian longhorned beetle). (Photo copyright of James E. Appleby, Department of Natural Resources and Environmental Sciences, University of Illinois, USA.)
188 Host trees of the tree downwards. As larvae mature they begin In its native range, A. glabripennis is found on to tunnel into the sapwood and heartwood, where 23 species of Populus (poplars) and at least 24 other they pupate, with adults emerging from large round broadleaved trees, including some species of Ulmus holes approx 1–1.5 cm in diameter (USDA, 2002). (elms), Salix (willows), and Acer (maples/sycamores), Consequently, the larvae not only inhibit nutrient as well as Aesculus hippocastanum (horse chestnut), and water flow within the tree, but also cause Malus pumila, M. alba (apples), and Prunus saliciana significant damage to the timber. The beetle is able to (Yang et al., 1995). In the northern USA, the beetle is attack healthy as well as stressed trees, and death of known to attack Acer (maples) species in particular, the tree often results after three to five years although it has also been found on a range of genera, (Smith et al., 2005). and is regarded as a specific threat to the maple sugar industry (USDA, 2002). Control Where infested trees are identified they are removed Distribution and destroyed; this is currently the main form of The native range includes China, Taiwan, Korea, control. The efficacy of some systemic insecticides is Japan, and parts of Southeast Asia. The beetle was being investigated, but none are currently used. first discovered in the USA in 1996 (Haack et al., There is potential to control the beetle using 1997a) and is present in New York City and entomopathogenic (insect-infecting) nematodes Chicago. Although not yet confirmed as present in such as Steinernema feltiae and S. carpocapsae, Europe, computer models predict that there is a since these can detect and infect larvae that lie below significant risk of importation and establishment on the bark (Fallon et al., 2004). There is also ongoing the continent (MacLeod et al., 2002). investigation of the potential of parasitoids, both those from the beetle’s natural range and those that Impacts/ecology attack related beetle species in the areas invaded. The adult beetles feed on leaves and twig bark, but The bethylid wasp Scleroderma guani and the do not cause significant damage. The larvae hatch colydiid beetle Dastarcus longulus may have from eggs laid under the bark and feed initially in the potential as control agents (Yang and Smith, 2001; cambium during the second and third instar or larval Smith et al., 2005). stages (458). Infestations usually occur from the top SHOOT BORERS 458 458 Larval galleries HYPSIPYLA GRANDELLA ZELLER under the tree bark (LEPIDOPTERA: PYRALIDAE) – MAHOGANY excavated by the larvae of SHOOT BORER Anoplophora glabripennis (Asian Description longhorned The adult moths have brown forewings and beetle). (Photo buff-coloured hind wings with a wingspan of copyright of approximately 2–4 cm. The forewings have a faint James E. Appleby, zigzag pattern and both forewings and hind wings Department of have dark veins. Larvae are reddish-brown in colour Natural Resources with large black spots, and have a reddish head and Environmental capsule with a black band behind (459). There are Sciences, University five to six instars (larval stages between moults). of Illinois, USA.)
INSECT PESTS OF SOME IMPORTANT FOREST TREES 189 Host trees Distribution These include a wide range of the family Meliaceae H. grandella is the main pest of mahogany in the (sub-family Swietenioideae), including the commercially New World, central and tropical South America, the important Swietenia (American mahoganies), Khaya Caribbean, and Southern Florida, while H. robusta (African mahoganies), Toona (Burma cedar), and is the main pest in Africa and the Asia/Pacific region Cedrela (American cedars). The larvae feed in the shoots (Horak, 2001). and fruits (Griffiths, 2001). Impacts/ecology 459 Larva of 459 The larvae tunnel in the developing shoots of young Hypsipyla trees, frequently killing the leading shoot, which grandella results in stunted, crooked growth (Griffiths, 2001); (mahogany shoot they may also feed under the bark. The larvae borer) on a host produce silk webbing in order to protect themselves tree. from natural enemies and to improve their local microclimate. The generations last one to two months, extending to five months if the larvae go into diapause (a period of dormancy) triggered by cool temperatures or drought (Griffiths, 2001). Tree death can result from repeated attacks (460). Some adults feed on the fruit of the tree, causing premature fall. 460 Damage to Control a stand of trees The impact of the shoot borer may be reduced by caused by silvicultural methods, including pruning on high- infection with quality sites to improve tree form, the production of Hypsipyla trees which recover rapidly after attack, and the use of grandella. nurse crops (Hauxwell, 2001). The use of chemical control agents is generally restricted to integrated pest management (IPM) systems or to nurseries, as they are not effective on their own (Wylie, 2001). Parasitoids 460 with the potential to control H. grandella have been identified, although further work on their biology is needed. Manipulation of native predatory ant populations, including Oecophylla spp. (weaver ants), Anoplolepis spp., and some Iridomyrmex spp., is thought to be worth exploration (Sands and Hauxwell, 2001). TOMICUS PINIPERDA LINNAEUS (COLEOPTERA: SCOLYTIDAE) – COMMON PINE SHOOT BEETLE Description The adults are cylindrical and range from 3.5 to 4.8 mm in length. They have a black head and thorax, with reddish-brown to black elytra (wing cases). The anterior edge of the elytra is ridged or armed. The larvae are legless with a white body and brown head, and may be up to 5 mm in length.
190 Host trees Impacts/ecology Various species of Pinus (pine) are the main host There are two phases of attack. Initially adult beetles, trees. When beetle populations are high, the adults which are associated with damaging blue stain fungi, may breed in Picea (spruces), Abies (firs), and Larix will attack weakened trees and recently cut stumps (larches) which occur in stands mixed with pine. and logs, using these as breeding sites. Volatile chemicals such as alpha-pinene are used to locate Distribution suitable material, and adults fly distances of up to The beetle occurs in Eurasia, North America (Illinois, 1 km to find it. Galleries are constructed for egg Indiana, Maryland, Michigan, New York, Ohio, laying, and also by the larvae in the inner bark and Pennsylvania, and West Virginia; and Ontario, outer sapwood (461). After pupation the adults Canada), Kunming, China, and Japan. It is found in emerge, leaving 2 mm diameter circular holes in the sympatry (occurring in the same geographical region bark. The second and most economically significant but without interbreeding) with T. destruens in France phase of attack then occurs. Beetles fly to the crowns and the Mediterranean region (Haack et al., 1997b). of healthy trees, where they carry out maturation feeding by boring into and hollowing out the centres 461 461 Galleries created of lateral shoots. The shoots bend, turn yellow/red, by Tomicus piniperda and break off (462, 463). Each adult can destroy up to (common pine shoot six shoots, reducing productivity by up to 20–40% beetle) under the bark and affecting tree form (Fagerström et al., 1978; of Pinus nigra Speight and Wainhouse, 1989; Haack et al., 1997b). (Corsican pine). Control One of the most effective methods of control is to remove potential brood material of cut trees and stumps from the site. Trap logs may be set to attract adult beetles, and subsequently destroyed. Potential biological control agents include the predatory beetle Thanasimus formicarius (L.) (Clerididae) and species of Rhizophagus (Rhizophagidae) (Haack et al., 1997b). A range of foliar chemical sprays including permethrin may be used to reduce beetle populations. 462 463 462 Pinus sylvestris (Scots pine) shoots attacked by Tomicus 463 Numerous fallen pine shoots excised by Tomicus piniperda showing shoot bending and yellowing of their piniperda feeding on the young shoots (see 462). needles.
INSECT PESTS OF SOME IMPORTANT FOREST TREES 191 BARK CHEWERS Host trees HYLOBIUS ABIETIS LINNAEUS (COLEOPTERA: Many species of conifers, particularly Picea CURCULIONIDAE) – LARGE PINE WEEVIL sitchensis (Sitka spruce), Pinus sylvestris (Scots Description pine), and Larix spp. (larches), act as hosts. The weevil is also a pest of several broadleaf tree species. The adult weevil is 9–14 mm long and dark brown, Some trees such as Pseudotsuga menziesii (Douglas with cream-coloured markings on the elytra (wing fir) and Picea abies (Norway spruce) are less cases) (464). The attachment of the antennae is at vulnerable after two seasons’ growth, whereas other the tip of the snout, near the mouthparts (Byers, species remain susceptible to attack (Rose and 1989). The adults live for up to four years and are Leather, 2003). nocturnal in habit (Heritage and Moore, 2001). A newly emerged adult is capable of reproduction Distribution within a month and eggs are laid underground, near The pine weevil occurs in western and eastern a newly felled pine tree (Rose and Leather, 2003). Europe. The larvae are pale and legless with a brown head capsule (465). Impacts/ecology This weevil is regarded as the most damaging and 464 economically important pest of restocked forests in Europe (Rose and Leather, 2003). The larvae feed 464 Adult of Hylobius abietis (large pine weevil) – note the under the bark of stumps where they do no economic prominent antennae attached to the tip of the snout. damage; this stage may last for two to five years. However, the adult Hylobius abietis is a significant pest, with two population peaks, spring and late summer, and therefore two periods of damage. In the UK, Hylobius is estimated to cost the Forestry Commission up to £2m per annum. Adults may live for four years and hibernate over winter, emerging in spring to feed. They feed on the bark and cambium in newly planted trees (466) – often at the base of the root collar in coniferous species and from the top of the stem in deciduous trees. Young trees are rapidly 465 466 466 Bark damage caused by the feeding of Hylobius abietis adult. 465 Legless larva of Hylobius abietis.
192 ring barked and can be killed in a very short time Host trees (Heritage and Moore, 2001). The impact of this pest The larvae feed on a very wide range of host may be very serious, with weevils likely to kill up to plants (over 500 species). Young larvae feed 50% of newly planted trees on a site if no treatment primarily on broadleaf trees including Quercus is applied (Heritage and Moore, 2001). (oaks), Populus (aspens/poplars), Betula (birches), Salix (willows), and Alnus (alders), whereas later Control instars can utilize a greater range of trees, Cultural control is not effective for H. abietis as its including conifers. populations increase rapidly and kill trees very quickly. Prophylactic chemical control is currently Distribution used in the form of permethrin sprays and dips The gypsy moth is a native to Europe, southern Asia, (Rose and Leather, 2003). Biological control is and northern Africa. Its distribution includes being attempted using nematodes which locate and British Colombia, Quebec, and Ontario in Canada, infect the host and rapidly develop inside it, and Utah, California, and Washington State in the resulting in septicaemia of the weevil caused by USA (Humble and Stewart, 1994). toxic bacteria (Xenorhabdus) accompanying the nematode. There are also some parasitoid wasps, Impacts/ecology mainly Braconid spp., which attack both the larvae When populations are high, the gypsy moth defoliates and adults. millions of hectares of forest and urban trees. Outbreaks occur in the USA every seven to eight years, DEFOLIATORS and can be three to four years in duration. The female LYMANTRIA DISPAR LINNAEUS (LEPIDOPTERA: moth lays a single egg mass on any available surface, LYMANTRIIDAE) – GYPSY MOTH including trees, rocks, fences, etc. The emerging larvae Description move to the tops of trees and are carried many miles on wind currents by ballooning on silk threads. The adult female moth is large, white, and flightless, Defoliation results in loss of productivity, and impacts although winged. The wings have a dark zigzag on the recreational use of forests (469), but mortality pattern and a span of around 5 cm (467). The male is only likely if other stress factors for the tree are in has a wingspan of approximately 3–4 cm. The larvae play (Humble and Stewart, 1994). The caterpillars have long hairs, and the final instars of the larvae are have histamine-carrying hairs, which can produce grey with five pairs of blue dots followed by six pairs strong skin and respiratory reactions, so they are of red dots along their backs (468; Humble and regarded as a major public health hazard. Stewart, 1994). 467 468 467 Adult of Lymantria dispar (gypsy moth) showing the 468 Larva of Lymantria dispar (gypsy moth) – note its long characteristic zigzag pattern on its wings. (Photo copyright hairs, which can cause an acute allergic reaction in humans. of Thérèse Arcand, NRS Canada, CFS Laurentian Forestry (Photo copyright of Thérèse Arcand, NRS Canada, CFS Center.) Laurentian Forestry Center.)
INSECT PESTS OF SOME IMPORTANT FOREST TREES 193 Control CHORISTONEURA FUMIFERANA CLEMENS Mechanical methods of control can be used in (LEPIDOPTERA: TORTRICIDAE) – EASTERN urban areas, and include removing egg masses from trees and the application of grease bands to trees to SPRUCE BUDWORM prevent larvae moving down to the ground to pupate. A range of chemical control agents such Description as permethrin and diflubenzuron (a synthetic The adult moths are greyish to copper-brown with a moulting agent) is effective in larger areas of wingspan of approximately 2 cm (470; Cerezke, outbreak. Biologically derived pesticides that are 1991). Females lay egg masses that are bright-green effective against gypsy moth (Sparks et al., 1998) and found on the underside of needles. The young include the bacterial toxin Bacillus thuringiensis larvae are very small and light-yellow/green, becoming var. kurstaki (B.t.k), neem, and Spinosad (produced brown with a dark-brown head capsule (471). By the by the soil actinomycete Saccharopolyspora sixth instar they range in size from 1.8 to 2.4 cm in spinosa). length. Pupation occurs in webbing in the tree foliage. 469 Host trees This is a major forest pest and attacks firs and spruces such as Picea glauca (white spruce) and Abies balsamea (balsam fir), and, less commonly, Larix (larches) and Pinus (pines). Distribution The spruce budworm is native to North America and is found throughout most of Canada and the northern USA. 469 Defoliation damage caused by Lymantria dispar (gypsy Impacts/ecology moth) to a stand of broadleaved trees. (Photo copyright of Spruce budworm larvae damage trees by mining the Claude Monnier.) current year’s needles and new cones. Older larvae will occasionally feed on mature needles. The female moths lay eggs in masses on the underside of needles in August, and the peak feeding period for larvae is April to June. Feeding results in defoliation, giving the trees a reddish-brown tinge. The leading stem may 470 471 470 Adult of Choristoneura fumiferana (eastern spruce 471 Larva of Choristoneura fumiferana (eastern spruce budworm). (Photo copyright of Thérèse Arcand, NRS budworm). (Photo copyright of Thérèse Arcand, NRS Canada, CFS Laurentian Forestry Center.) Canada, CFS Laurentian Forestry Center.)
194 be killed, affecting tree shape. Seed production may be spray (Cerezke, 1991; Unger, 1995). Pheromone reduced and productivity is lowered (472). Where (attractant chemical) traps are used to monitor trees are susceptible (due to age or condition), population and dispersal trends of adult moths, mortality may occur after several years of defoliation together with aerial surveys of defoliation. B.t.k is (Cerezke, 1991). used as a biological control agent and is optimally applied when the larvae are 10–15 mm long, Control usually in June. Cultural practices may help to reduce impact, and include planting even-aged, low-density MALACOSOMA DISSTRIA HÜBNER stands, maintaining vigorous growth, and early (LEPIDOPTERA: LASIOCAMPIDAE) – FOREST harvesting (Unger, 1995). In Canada, tebufenozide (Mimic®) is used as an aerial chemical control TENT CATERPILLAR 472 472 Damage to a Description young tree shoot The adults are pale yellow to reddish-brown with caused by a wingspan of approximately 2.5–4.5cm. The Choristoneura forewings are marked with two diagonal lines, which fumiferana may form a dark band (473). The larvae are dark (eastern spruce brown and hairy with blue to blue-black sides, budworm). reaching 4.5–5.5 cm by the last instar. The dorsal (Photo copyright surface is marked with whitish, keyhole-shaped of Claude spots bordered by two orange lines. There is a Monnier.) brightish blue band along the side of the larva (474; Wood, 1992). Host trees The larvae feed on deciduous trees such as species of Populus (poplars/aspens), Salix (willows), Alnus(alders), Betula (birches), Prunus (plums), and Quercus (oaks). Distribution The forest tent caterpillar is native to and widespread in North America. 473 474 473 Adult of Malacosoma disstria (forest tent caterpillar). 474 Larva of Malacosoma disstria (forest tent caterpillar). (Photo copyright of Thérèse Arcand, NRS Canada, CFS (Photo copyright of Thérèse Arcand, NRS Canada, CFS Laurentian Forestry Center.) Laurentian Forestry Center.)
INSECT PESTS OF SOME IMPORTANT FOREST TREES 195 Impacts/ecology for example that by Icterus galbula (northern In spite of the name, the larvae do not build a tent, oriole), has a significant impact on densities (Parry but create a mat on which they feed in groups. et al., 1997). Outbreaks of tent caterpillar occur about every 10 years and can last anything from three to seven TERMITES years. The larvae begin to feed at budburst and may kill the buds, causing trees to grow with poor form Termites are social insects, living in colonies, with and to lose twigs and branches. Feeding by older several castes or body forms specialized to larvae may result in complete defoliation, reducing perform specific tasks, such as soldiers, workers tree productivity and occasionally causing (absent in primitive families), and reproductively mortality (475). However, death is only likely if a active, winged (alate) forms. Termites live in tree succumbs to drought, other pests, or disease, or nests which are constructed differently across after several subsequent years of defoliation groups; in the genus Coptotermes, nests are built (Wood, 1992). underground (in living/dead trees or in man-made structures) from a combination of soil, chewed Control wood, saliva, and faecal material (collectively Egg masses and larvae are often cut out of trees in called carton). Soil is often compacted around the urban areas. In forest situations infestations may outside. be left to take their course as populations are eventually reduced by adverse environmental COPTOTERMES FORMOSANUS SHIRAKI conditions (e.g. frost), starvation, and natural (ISOPTERA: RHINOTERMITIDAE) – FORMOSAN enemies. However, B.t.k and a range of chemical pesticides, including permethrin, may be used SUBTERRANEAN TERMITE AND to control the caterpillars. An NPV (nuclear polyhedrosis virus) is a key natural agent of COPTOTERMES ACINACIFORMIS FROGATT mortality in the wild, and is currently being (ISOPTERA: RHINOTERMITIDAE) explored for its potential as a control agent (Frank Description and Foltz, 1997). Over 80% of the tent caterpillar pupae may be parasitized and killed by Soldier termites are often used for identification Sarcophaga aldrichi Parker (tachinid flesh fly), of Coptotermes species because they are but Hymenopteran parasitoids are unlikely to proportionately numerous in the colonies. control populations. However, bird predation, Coptotermes formosanus soldiers have oval orange- brown heads and black jaws or mandibles. The body of the soldier is creamy-white and up to 6.5 mm in length (476). The winged forms (alates) are yellow-brown, large in size at 12–15 mm long, 475 476 475 Defoliation damage caused by Malacosoma disstria 476 Termite soldiers of Coptotermes formosanus showing (forest tent caterpillar) to a stand of broadleaved trees. their large mandibles (Photo copyright of Nan-Yao Su, Fort (Photo copyright of Pierre Therrien.) Lauderdale Research & Education Center, University of Florida, USA.)
196 478 477 477 Alate (winged reproductive) termite of Coptotermes formosanus showing hairs on its wings. (Photo copyright of Nan-Yao Su, Fort Lauderdale Research & Education Center, University of Florida, USA.) 478 Specimen of Eucalyptus sp., the heartwood of which has been hollowed out (piped, note the debris) by an attack by termites (Coptotermes acinaciformis) near Darwin, Australia. It is estimated that up to 50% of eucalypts in this region suffer from termite infestation. (Photo copyright of Bryan Bowes.) and have many small hairs across the wing surface Distribution (477; Scheffrahn and Su, 1994). Coptotermes C. formosanus is thought to be endemic to southern acinaciformis (tree piping termite) is of a similar size China and is currently found in Japan and Hawaii to C. formosanus (5–6.6 mm long), and has a (Tamashiro and Su, 1987), South Africa, and Sri teardrop-shaped head and tapering toothless Lanka. In the USA it has been found in Alabama, mandibles. California, Florida, Georgia, Hawaii, Louisiana, Mississippi, North and South Carolina, Tennessee, Host plants and Texas. (http://creatures.ifas.ufl.edu/urban C. formosanus attacks a wide range of plant /termites/formosan_termite.htm). species, living and dead, as well as man-made constructions and non-cellulose-based materials; it C. acinaciformis is native to Australia, and has is often found in damp locations (Cabrera et al., been found in isolated colonies in New Zealand, but 2001). C. acinaciformis attacks and hollows out to date these colonies have been successfully (or pipes) live trees, and Eucalyptus species are eliminated. known to be particularly susceptible (478). It also attacks human constructions.
INSECT PESTS OF SOME IMPORTANT FOREST TREES 197 Impacts/ecology 1999) and therefore have the potential to be regarded The large size of Coptotermes formosanus as insect pests. The list of insect pests is rising steadily. colonies (there may be as many as several million Given that a greater area of global forest cover is individuals), makes this species a highly destructive being exploited every year and that there is increased pest. Most infestations of man-made structures result movement of people, materials, and insects around from the termites colonizing upwards from their the globe, it is likely that the list will continue to grow. underground nests; however they are also able to The future for insect pest control lies in an IPM construct aerial nests when conditions are appropriate strategy that takes into account the system in which (Su and Scheffrahn, 1987). Infestation is often quite the insect operates, clear assessment of actual damage advanced before signs such as soft floorboards and to the forest, and a range of effective and appropriate blistered tree surfaces are apparent. C. formosanus monitoring and control methods, including cultural, is regarded as one of the most significant pests in biological, and chemical methods. the USA (http://creatures.ifas.ufl.edu/urban/termites/ formosan_termite.htm). C. acinaciformis is the most economically important termite in Australia, causing significant damage to various species of Eucalyptus. It is also a major pest of urban forestry and human constructions. C. acinaciformis can build subsidiary nests, connected by underground galleries or tunnels to the main nest, and has an extensive foraging range. Control Aggregation and control – one of the main methods of controlling termites – involves encouraging them to aggregate at a food source or ‘bait’, and then eliminating them using chemicals such as arsenic trioxide or metabolic inhibitors such as sulfuramid (Peters and Fitzgerald, 2005). Physical barriers and chemical barriers, such as a soil drench of pyrethroid chemicals, may be effective, although chemicals are short-lived and application must be repeated. The use of more resistant building timber such as Callitris (cypress pines) can be useful (Peters et al., 2005). The role of entomopathogenic fungi in the control of Coptotermes formosanus is being investigated. Fungi may be of use in an IPM system, when combined with bait systems (Wright et al., 2004). CONCLUDING REMARKS Insect herbivores are part of a complex ecological system, in which they utilize plant material and are themselves exploited by predators and parasites. A great many groups of insects are herbivorous – over 50% of extant species feed on plants (Speight et al.,
198 SECTION 5 FOREST ECOLOGY AND MANAGEMENT, TREE SURGERY, PROPAGATION, AND CONSERVATION CHAPTER 10 General forest ecological processes Peter A Thomas INTRODUCTION above-ground portion of the woody plants. Biomass above ground increases from the northern boreal Wooded land currently covers between 30% and forest southwards towards the tropics, starting from 35% of the world’s land surface (depending on what very low levels at the Arctic tree line, and reaching in is counted as forest), or around 39–45 million km2 (FAO, 2003). Ecologists often distinguish between 479 woodland and forest (479, 480). Woodland is a small area of trees with an open canopy (usually defined as the canopy giving less than 40% cover, that is 60% or more of the sky is visible) so that plenty of light reaches the ground, encouraging other vegetation beneath the trees. By contrast, a forest is usually considered to be a relatively large area of trees forming a closed, dense canopy. For simplicity’s sake, and because the underlying ecological processes at work are the same, in this chapter the term forest will be used to mean any wooded land. SIZE AND GROWTH 479 A dense temperate rain forest in the Olympic Peninsula, Washington State, USA. It is composed The most obvious factor that separates forests from mostly of Pseudotsuga menziesii (Douglas fir), Tsuga other types of habitat is the large weight or mass of heterophylla (western hemlock), and Thuja plicata organic material present, referred to as the biomass (western red cedar). (or sometimes the standing crop). In most forests, more than 85% of the biomass is contained in the
GENERAL FOREST ECOLOGICAL PROCESSES 199 excess of 940 t ha–1 in the Amazon basin. However, amount of extra carbon that is being pumped into the there are exceptionally large forests outside the atmosphere – usually approximately 25% extra tropics, notably the temperate forests of the Pacific forest globally. However, such an estimate is blatantly Northwest of North America. These include stands wrong. When a forest is mature it reaches an of huge Psuedotsuga menziesii (Douglas fir, 479), approximately steady state of mass, where NPP is reaching 1,600 t ha–1, and Sequoia sempervirens balanced by an equal loss in biomass through (coastal redwoods), the tallest trees in the world, decomposition. At this point, the productivity of the which have a biomass of up to 3,450 t ha–1 just in the whole forest (the net ecosystem productivity – NEP) trunks. Below-ground biomass in roots is signifi- drops to near zero. Thus, it is only young forests that cantly less (Jackson et al., 1996), averaging 29 t ha–1 are carbon sinks; once forests are mature they in boreal forests, 40–42 t ha–1 in temperate and become carbon neutral. In reality, temperate and tropical deciduous forests, and 49 t ha–1 in tropical northern forests globally are a net sink of carbon, but evergreen forests. this is primarily due to expansion of the amount of forest due to reforestation (Beedlow et al., 2004). Biomass is a static measure of how much mass there is at any one time, with no indication of how LIGHT quickly new growth is being added or lost, and so gives little insight into how the forest is functioning. Trees have evolved as a life form to outcompete More useful are estimates of the productivity of the their neighbours for light by growing tall, so forest, i.e. how much new material is being added per producing dense forests that inside are darker, year, described as net primary productivity (NPP). more humid, and less prone to extremes of This can vary from as little as 1 t ha–1 y–1 in cold temperature variation than outside. In temperate boreal forests, to over 30 t ha–1 y–1 in tropical forests at least, it is usually possible to recognize rainforests, with an average of 7–12 t ha–1 y–1 in four reasonably distinct layers (480). At the top is temperate forests. However, a maximum of 36.2 t the tree canopy, normally 5+ m above ground. ha–1 y–1 has been recorded in the Pacific Northwest Below are the shrub layer (<5 m), the field or herb from a 26-year-old forest of Tsuga heterophylla layer of herbaceous plants and short woody plants (western hemlock). These figures have sometimes such as brambles, and the ground or moss layer of been used to calculate how much additional forest mosses and liverworts, lichens, and algae. Each needs to be planted to soak up (sequester) the huge layer blocks sunlight so that a dense layer may 480 480 Open woodland at Needwood Forest, England. The sparse canopy of Fraxinus excelsior (ash) and Tilia spp. (lime) standards allows abundant light to reach the shrub layer of coppiced Corylus avellana (hazel) and, on the ground, a mixed field layer dominated by Hyacinthoides non-scripta (bluebell). A sparse ground layer of mosses is also present.
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