Feeding Groups

Insects can be assigned to feeding groups based on the part of the tree they attack and the method of feeding they use. In forest habitats, these feeding groups are:

A. Insects that feed on cones and seeds

B. Insects that feed on shoots and tips

C. Insects that feed on foliage

D. Insects that feed on the trunk and large branches

E. Insects that feed on roots

In the following discussion, species have been selected from each feeding group to demonstrate the great diversity of insect adaptations to their tree hosts. An example is given from each of the major orders of insects associated with each feeding group. The examples are taken mostly from the Old (Palearctic) and New (Nearctic) World fauna that have received the greatest scientific attention to date. We select examples based on insect orders, knowledge base, and a diversity of biological attributes, all within each feeding group. This approach should give the greatest insights into the mechanisms of insect interactions with their host trees. Thus, the reciprocal responses of the host and the insect are considered at the level of the individual tree and how this affects forest successional patterns.

Insects That Feed on Cones and Seeds

Important species of cone and seed insects are found in the Coleoptera, Diptera, Heteroptera, Hymenoptera, and Lepidoptera. They damage the reproductive structures of trees, resulting in reduced seed production. These insects may feed on the seed itself or on structures associated with the seed. The number of insects feeding in conifer cones is very large when the small size of the feeding substrate is considered. For example, in western Europe, cone- and seed-feeding insects number some 59 species from 30 genera, 13 families, and 4 orders.

Conophthorus spp. (Cone Beetles); Coleoptera: Scolytidae (Bark and Ambrosia Beetles) Species in this genus colonize only pine species in North America. Morphological divergence is greatly reduced and therefore, species separation is difficult. Thus, many species were described based on their host associations. Recent studies of pheromone components and DNA are helping to establish the status of these sibling (closely related and difficult to distinguish) species.

Except for the time spent between emergence from the brood cone and colonization of a new cone, the entire life cycle is spent within the cone. A single female tunnels into a new cone in the spring through the cone scales near the supporting stem (as in the "hard" pines, e.g., ponderosa pine) or into the supporting stem of "soft pines" (e.g., eastern white pine, Pinus strobus). She excavates a gallery along the cone axis and deposits eggs in the gallery wall. The gallery is packed with frass, which is composed of uneaten fragments of cone tissues and excrement. The male mates with the female inside the cone prior to gallery elongation. One female can infest more than one cone. In general, infested cones from the soft pines fall prematurely from the tree, while infested hard pine cones remain attached to the branch until they drop in the fall. Beetle attacks cause the death of the cone before and after the cone reaches full size. New brood adults can overwinter in the cone or emerge and mine twigs on living trees (C. ponderosae on sugar pine, Pinus lambertiana) or mine first-year cones (C. radiatae on Monterey pine) and spend the winter as nonreproducing adults.

Other insect species enter the galleries of the cone beetle. The dry twig and cone beetle, Ernobius punctulatus (Anobiidae), feeds in the drying cone tissues that follow cone beetle feeding activity. A parasitic wasp (Bethylidae: Cephalonomia species) enters the cone beetle tunnel to lay eggs. The wasp larvae then feed on the cone beetle larvae.

Contarina oregonensis (Douglas-Fir Cone Midge); Díptera: Cecidomyiidae (Gall Midges) This midge infests cones of Douglas-fir in California, Oregon, Washington, and British Columbia. Eggs are laid in young female cones as they open for pollination in the spring. Larval feeding tunnels stimulate gall formation on the seed coat, which destroys the seed. Several galls may form on the seed coat. In the fall, the mature larvae leave the cone and fall into the litter beneath the tree where they spend the winter. There is one generation each year; however, some individuals remain in a resting stage in the litter for more than 1 year. Arrested development, termed diapause, is common in insects that infest cones and seeds. This mechanism permits the insects to survive through periods when few cones are produced. Many other species of insects can inhabit these cones with this midge, including moths, beetles, and wasps.

Megastigmus spermatrophus (Douglas-Fir Seed Chalcid); Hymenoptera: Torymidae (Torymids) Most species in this family are parasites of gall-forming insects, which are usually other wasps or flies. However, in the Douglas-fir seed chalcid, the female inserts her long ovipositor through the cone scales and into the seed where she deposits an egg. The larva develops entirely within the seed coat. The seed coat continues to develop normally. In the spring of the following year the adult cuts a round hole in the mature seed coat and emerges to begin a new generation. Adult emergence may be delayed 2 to 3 years. Cones are harvested in the fall to extract seed for future plantings. Infested seeds cannot be distinguished from unin-fested seeds by external appearance. This species has been introduced into Europe and has become a serious pest in plantations of Douglas-fir. Parasites reared from infested cones in western North America have been introduced into France to help reduce populations of this seed chalcid. M. speculatris was also introduced from North America and now infests cones of Siberian fir, Abies sibirica and Nordmanns fir, A. nordmanniana in Europe.

Leptoglossus occidentalis (Western Conifer Seed Bug); Heteroptera: Coreidae (Coreid Bugs) This insect is named the "leaf-footed bug" because the tibiae of the hind legs are broad and flat while the tibiae of the first two pairs of legs are tubular in shape. This insect feeds on the seed of many conifer species, including Douglas-fir, incense cedar, Libocedrus decurrens, and ponderosa pine. This bug sucks plant juices, which results in damage to the seed. The eggs are barrel-shaped and are deposited in rows on the needles. The adults overwinter in protected areas including inside buildings. The broad host range of this species favors survival during years with low cone production in one or more host species.

Cydia strobilella (Spruce Seed Moth); Lepidoptera: Tortricidae (Tortrix Moths) The female spruce seed moth oviposits between the cone scales of spruce flowers. After hatching, the larvae feed on the central stalk of the cone before moving to the developing seeds. Each larva consumes several seeds to complete its development. Fourth instars return to the cone stalk to overwinter and may remain in extended diapause for 2 or more winters. Higher than average summer temperatures result in extensive cone production in the following year and also stimulate the larvae of spruce seed moth to break diapause. However, even in good years, not all larvae within a cone break diapause. In some European seed orchards, over 50% of cones may be damaged by this species.

Insects That Feed on Shoots and Tips

Insects feeding in or on shoots and tips are found in the orders Coleoptera, Diptera, Hemiptera, and Lepidoptera. These insects may feed externally or internally on these structures. In addition, they may induce the tree to produce a gall as a result of their feeding activities. This gall may have nutritional as well as protective benefits to the insect. Some insects that feed externally gain protection by, for example, producing a spittle mass, as in spittlebugs, or wax, as in woolly aphids. Feeding may be on foliage in addition to the shoot and/or tip.

T. piniperda (Pine Shoot Beetle); Coleoptera: Scolytidae (Bark and Ambrosia Beetles) In spring, adults of the pine shoot beetle emerge from overwintering sites in the "duff," the needle and bark litter on the ground. They also overwinter in short galleries below the litter layer. They attack trees weakened by root disease (e.g., annosus root disease caused by Heterobasidion annosum) and broken and cut trees, but are also capable of killing apparently healthy trees. There can be two or more early emergence periods that result in "sister" broods. This bark beetle infests several pine species, but Scots pine, Pinus sylvestris, is its principal host in Europe. The female excavates an egg gallery in the phloem, which is parallel with the grain of the wood. The male mates with the female in the egg gallery. She cuts niches in the gallery wall and deposits one egg in each niche. Larvae then tunnel laterally from the egg gallery. The larval gallery gradually becomes wider as the larvae develop. Pupation occurs in the early summer and emergence occurs in midsummer. These adults fly to living trees and tunnel into the axils of young shoots, which causes the shoots to die and break off. The feeding that occurs in the shoots is necessary for maturation of the gonads. The beetles emerge from the dying shoots and attack host material as described above. This habit has been termed "maturation feeding." Death of the terminal shoot causes flattened canopies. The pine shoot beetle was discovered in Ohio in 1992 and has since spread to adjoining states and Ontario, Canada. It infests eastern white pine and Scots pine Christmas tree plantations.

C. piniinopsis (Gouty Pitch Midge); Diptera: Cecidomyiidae (Gall Midges) The gouty pitch midge feeds on a large number of pine species in North America. Only the hard pines are infested. It is usually found feeding on young, open-grown pines in both natural stands and plantations. Damaged shoots are first observed in early summer, when new shoots droop and turn yellow and then red. Repeated infestations in northern California slow tree growth and eventually kill the tree. Heaviest infestations occur on ponderosa pines exhibiting sticky twigs while lightest infestations occur on dry, powdery stems, suggesting that some genotypes are resistant to the feeding-induced damage caused by this midge. The bright red larvae overwinter in small resinous pits beneath the bark. In spring larvae migrate to the needles where they pupate. There is one generation per year. Dead shoots resemble frost-damaged tips.

Adelges cooleyi (Cooley Spruce Gall Adelgid);

Homoptera: Adelgidae (Adelgids) The Cooley spruce gall adelgid is distributed from coast to coast in North America where it infests many species of spruce, including Sitka, Picea sitchensis, Engelmann, P. engelmannii, white, P. glauca, blue, P. pungens, and Brewer, P. brewerana. In the west, Douglas-fir is the alternate host. It was introduced into Europe along with Douglas-fir. In Great Britain, it is named the Douglas-fir woolly aphid. Infestations are also found on Sitka spruce, which has also been introduced into Europe as a plantation species. This aphid has a complex life cycle. When both hosts are present, there can be six life stages in addition to eggs and crawlers. The life cycle, which includes all life stages, is completed in 2 years. Immature females overwinter under bark scales near the tips of spruce twigs. In the spring, they develop into mature females, termed stem-mothers. They each lay up to several hundred eggs under a white cottony, waxy mass. The newly hatched nymphs feed at the base of needles. Light green to purple elongate galls are formed that enclose the nymphs. A chamber at the base of the needle may contain 3 to 30 wingless adelgids, which are covered with a white wax. A few winged adelgids may also be produced. The galls vary between 12 and 75 mm in length. After the nymphs leave the gall, it turns brown and hardens, often remaining on the tree for many years. Infestations kill branch tips, which stunts and deforms the trees. Nymphs transform to adults on needles and fly to Douglas-fir and lay eggs on newly developing needles, shoots, and cones. Infested needles are twisted and chlorotic and can drop from trees in large numbers. No galls are formed on Douglas-fir. Winged adults produced on this host fly back to spruce to oviposit. Where each host occurs independent of the other, continuous generations are produced on one host. In areas of western North America where spruce is rare or absent, two parthenogenetic generations are produced on the needles of Douglas-fir in one season. No galls are produced on spruce where Douglas-fir is absent.

Rhyacionia buoliana (European Pine Shoot Moth); Lepidoptera: Olethreutidae (Olethreutid Moths) This moth occurs throughout Europe where it feeds on almost all pine species. It was first discovered in America in New York State in 1914 and later was found in British Columbia, Washington, and Oregon. It has not spread into other western North American states. This moth is a serious pest of pine plantations in Europe and North and South America. Recently, it has spread rapidly throughout the extensive Monterey pine plantations in Chile. Lodgepole pine planted in France are often heavily damaged, as are red pine, Pinus resinosa, plantations in the states and provinces that border the Great Lakes in North America. In summer, the female lays yellowish, disk-shaped eggs in rows on needles as well as under and on bud scales and on small twigs. The larvae hatch from the eggs and feed on the needles where they are attached to the twig. The larvae then spin a silken web between the needle sheath and the twig. The web is usually coated with resin. The larvae tunnel into the bud, which produces a resinous crust. These larvae overwinter under this crust or within the bud. In the spring, they mine another bud and then feed on the base of needles. They pupate within mined shoots, but the pupal skin protrudes from the shoot so that adult emergence is not impeded by the shoot or resin. The mined shoots are killed and buds formed later produce new shoots that grow around the dead shoot. Repeated infestations produce a bush-shaped tree. A crook is formed on the main stem when the terminal bud is killed.

Insects That Feed on Foliage

The foliage is the predominant photosynthetic part of the plant. Loss of foliage results in loss of photosynthetic area with resulting reduction in the production of carbohydrate. Defoliation may be tolerated in some species of trees, but less so in others. Successive cycles of defoliation, however, are rarely tolerated and often result in tree death, usually in association with other insect groups. Insects that feed on the foliage of trees are predominantly found in the orders Coleoptera, Diptera, Hemiptera, Lepidoptera, and Orthop-tera. Most of these feed externally on the foliage, though some mine into the foliage for part or all of their feeding stage. Various larvae in the Coleoptera, Diptera, and Lepidoptera mine the leaves of broadleafed plants, including trees. This activity protects the insect from desiccation and from predators. Many Hymenoptera induce galls to form on leaves as a result of oviposition into the leaf. These galls can take on many shapes and all protect the developing larva from desiccation and predation. Additional protection from abiotic and biotic effects may be provided by webbing to hold needles together, leaf rolling, or producing a silken nest in which insects congregate for protection from predators, as in the case of tent caterpillars.

Pyrrhalta luteola (Elm Leaf Beetle); Coleoptera: Chrysomelidae (Leaf Beetles) The elm leaf beetle is a native European chrysomelid beetle. It infests all species of elm. This beetle often causes severe growth loss, which weakens trees and, in turn, makes them susceptible to invasion by bark beetles and the Dutch elm disease fungus, which they carry into the tree.

The elm leaf beetle overwinters as an adult in sheltered, dry habitats, such as inside houses and barns and under loose bark on trees. They can become a nuisance in fall and spring as they enter and leave hibernation sites inside residences. They can be active in houses during the winter. Adults emerge from overwintering quarters and fly to elms at the time leaves are beginning to emerge from buds in spring. They lay eggs (up to 800 per female) in groups or irregular rows, along the major leaf veins on the underside of leaves and on nearby twigs. Larvae emerge from eggs in late spring and feed on the underside of leaves, causing skeletonization, where small veins are visible between the major lateral veins that emanate from the midrib. The later instars consume some of the area between these lateral veins. The leaves soon become desiccated and turn brown. At high densities, the entire tree turns brown and appears to be dead. However, in late summer some trees refoliate with smaller leaves. Repeated infestations can kill trees. Two or more generations are produced each season. This insect was introduced into the United States in the late 1890s and has since spread to the west coast. It feeds on all species of elm; however, American elm, Ulmus americana, and the introduced Siberian, U. pumila, and European elms are severely damaged by this beetle. Introduction of parasites into the United States has met with mixed control results.

Elatobium abietinum (Green Spruce Aphid); Homop-tera: Aphididae (Aphids) The green spruce aphid feeds on native and introduced spruce species in Europe and frequently occurs in forest nurseries. Introduced North American species such as Sitka spruce are most susceptible, whereas most European species, with the notable exception of Norway spruce, Picea abies, are less suitable as hosts. In parts of Europe a sexual as well as an asexual stage is found, and overwintering occurs as eggs. Elsewhere, such as in Great Britain, the sexual stage is absent, and parthenogenetically reproducing females remain on the needles during the winter. In North America, where this aphid is presumed to be an introduction, only the parthenogenetic form is known. Most feeding occurs when amino acid levels in the leaves are high in the winter and early spring. In late spring/early summer, longer day length coincides with a drop in amino acid levels, and winged individuals (alatae) are produced, which disperse to new hosts. Most outbreaks coincide with mild winters. The nymphs and adults feed on the lower side of older leaves where they use piercing mouthparts to access the phloem sap. They may completely defoliate trees and attack current-year needles later in the year.

Pristophora erichsonii (Larch Sawfly); Hymenoptera: Tenthredinidae (Sawflies) The larch sawfly is a native European species. It was first recorded in North America in 1880, in Alberta, Canada, in 1930, in Oregon in 1964, and in Alaska in 1965. Some strains are now believed to be native to North America. The prepupal stage overwinters in a cocoon in the ground or duff under defoliated trees. Adults emerge in spring and as late as late summer depending on location. They lay eggs in rows under the bark of newly elongating shoots. This oviposition behavior causes the shoots to droop. The common name of this family is derived from the saw-like ovipositor that females use to cut slits into the bark where they lay eggs. This species reproduces parthenogenetically. Larvae emerge from eggs and feed on the needle margins and then move to the older needle clusters. They feed gregariously and eat most of the needles on one shoot before moving to another. Between summer and fall, mature larvae drop to the ground and spin a paperlike cocoon in the forest litter. Here, they enter a diapause where they spend the winter. However, some individuals overwinter 2 or 3 years.

Successive defoliation results in tree mortality, especially eastern, Larix laricina, and western larch, L. occidentalis, in North America, and European larch L. decidua. It has been most destructive in the states around the Great Lakes. Where black spruce, Picea mariana, and eastern larch co-occur, black spruce is favored in succession because the larch sawfly kills both larch seedlings and mature trees. In the Alps, when aphid densities are high, larch sawfly densities are low, due in part to predation by ants that are tending aphids seeking their honeydew. Biological control efforts in the western United States have been successful in lowering the overall average densities of the larch sawfly. This program is considered one of the most successful biological control programs aimed at a widely distributed forest insect. During studies of the parasitoids of this sawfly, encysted larvae were discovered in certain populations. A cellular layer surrounding the larva is produced by the hemolymph (phagocytosis), a rare phenomenon in insects.

Lymantria dispar (Gypsy Moth); Lepidoptera: Lymantriidae (Tussock Moths) The gypsy moth is an important defoliator of hardwood trees in Europe and North America. It was introduced into Massachusetts in 1869. Since then it has slowly spread southward to North Carolina and westward into Wisconsin. Small populations have been eradicated repeatedly in California, Oregon, Washington, and British Columbia. In North America, massive defoliation has occurred over thousands of hectares. This species feeds on over 50 tree species, including mostly hardwoods, such as apple, beech, basswood, Tilia americana, elm, hornbeam, oak, poplar, and willow. During outbreaks, larch and pine are consumed. In southern Europe and northern Africa, outbreaks occur at irregular intervals. In North America, considerable resources are directed toward control and slowing the spread to uninfested areas. The Asian form of this species has been found in the western United States and Canada, but apparently has not become established.

In spring, larvae emerge from overwintering egg masses, at about the time oak leaves emerge from buds. The young caterpillars move to the tops of trees where they feed on the bases of young leaves or they chew small holes in the leaf surface. They also drop from trees on silken threads and are windblown. Older larvae feed mostly on the leaf margins. The larger veins and midribs of leaves are usually not consumed. At high densities (tens of thousands per hectare), the larvae literally "eat themselves out of house and home." They move to new areas in search of food or pupate, often giving rise to small adults. The larvae are very colorful, exhibiting six longitudinal rows of tubercles. On the thorax and first two abdominal segments the tubercles are blue and on the rest of the abdomen they are red. Larvae crawl down the tree and aggregate in sheltered places. At night they crawl up the tree to feed. In June, larvae pupate on many substrates, including rocks, limbs and trunks of trees, picnic tables, automobiles, and forest debris. In North America, females are flightless, and after emergence from the pupae they crawl a short distance and emit a sex pheromone that attracts male moths. After mating, the females lay eggs in a mass and cover them with larval hairs and a frothy substance. These egg masses may be transported on vehicles or on lawn furniture and rocks that are moved inside vehicles. This means of dispersal gave rise to the common name, "gypsy" moth. The eggs overwinter in a diapause state. The Asian gypsy moth is of major concern because the females are excellent fliers. It has frequently been intercepted on the west coast of the United States, and flying females enhance the dispersal of this species if it becomes established. Many natural enemies of this moth, including parasitoids, predators, and protozoa, have been collected in Europe and introduced in North America. These biological control studies are among the earliest attempts to use indigenous natural enemies to lower densities of an introduced pest.

Coleotechnites milleri (Lodgepole Needleminer); Lepidoptera: Gelechiidae (Gelechiid Moths) The lodgepole needleminer has a unique life history in the Sierra Nevada of California. The adults are synchronized to emerge in midsummer in odd-numbered years. Adults are not known from even-numbered years. However, another population of this species (or an undescribed species) that occurs in a nearby mountain range in western Nevada also has a 2-year life cycle, but adults emerge in even-numbered years. Another undescribed species feeds on lodgepole pine in central Oregon, but this species has a 1-year life cycle.

After mating, females lay eggs in mined needles or on branches near the needle fascicles. First instars enter near the tip of a single needle and remain in the mine through the winter. During the even-numbered season, fourth instars mine several needles and overwinter in a single, mined needle. In spring of the odd-numbered years, fifth instars feed on many needles and finally pupate in a mined needle.

The most severe outbreaks occur in mature lodgepole pine stands and occur over large areas. In the high-elevation forests (4000 m) near Yosemite National Park, many outbreaks have been recorded, some lasting 16 to 18 years, before returning to low densities. There does not appear to be a regular pattern to these epidemics. Severe defoliation weakens trees, which predisposes them to infestation by the mountain pine beetle, Dendroctonus ponderosae. Thus these two insect species convert these extensive mature to overmature lodgepole pine forests to a young forest of lodgepole pine. The Sierra Nevada form of lodgepole pine, Pinus contorta var. murrayana, produces largely nonserotinous cones that are opened by sunlight. Furthermore, lodgepole pine is moderately shade tolerant and can thus regenerate in the understory of larger trees. When needle miner populations increase to epidemic levels, young trees are released to grow and seeds germinate to produce new seedlings.

The decline of outbreaks has been attributed to rain, and often snow, during the mating period, abundance of parasites and predators, and reduction in amount of host foliage for colonization. The biology of host-specific and generalist parasites has been investigated in an attempt to determine the basis for the 2-year synchronized life cycle.

Diapheromera femorata (Walkingstick); Phasmatodea: Heteronemiidae (Walkingsticks) Walkingsticks are very unusual insects because, as their name signifies, they are long, slender, and oval and thus resemble a stick or defoliated branch. Most species are wingless. Their body color varies from gray to green to brown to red, which is similar to the leaves and branches on which they are feeding. Through this camouflage they may escape predation from birds. In late summer and fall they deposit their hard, seed-like eggs directly on the ground or by dropping them from trees where they are feeding. At high population levels, the large number of eggs falling on the forest floor literally sounds like rain. Most young hatch during the following spring throughout the southern United States, whereas in the northern portion of its range, the young hatch in the second spring. Occasionally, severe outbreaks occur in the northern United States that last several years, causing extensive tree mortality. Two defoliations can occur during the same season. They feed on a variety of hardwood species, including cherry, basswood, birch, aspen, elm, hickory, oak, locusts, and dogwood.

Insects That Feed on the Trunk and Large Branches

Insects that feed on the trunk and larger branches of trees are found in the Coleoptera, Hemiptera, Hymenoptera, Isoptera, and Lepidoptera. Insects that tunnel into the trunk and large branches of trees may exploit two resources: they may derive most of their nutrition from the phloem, or they may tunnel into the wood and derive their nutrition from the xylem. Insects that do not tunnel into the host, such as scale insects, are sucking insects and use nutrients in the xylem or phloem depending on the species. Many insects that infest the wood of trees, such as termites, serve as primary decomposers of the woody material.

D. ponderosae (Mountain Pine Beetle); Coleoptera: Scolytidae (Bark and Ambrosia Beetles) The mountain pine beetle is one of the most important forest insects in North America. It is widely distributed in many pine species throughout western North America, from southeastern Alaska to northern Baja California, and eastward through the Yukon territory in Canada and the Rocky Mountains of both the United States and Canada to the Black Hills of South Dakota. The eight-toothed spruce bark beetle, Ips typographus, shows a similarly wide distribution in Europe and Asia but it has a more limited host range than D. ponderosae. During outbreaks, this bark beetle kills millions of trees over large areas. It infests some of the most widely distributed and important timber-producing species, including ponderosa pine, western white pine, P. monticola, and sugar pine. Extensive mortality of ponderosa pine at the turn of the 19 th century in the Black Hills of South Dakota attracted public attention that led to the establishment of the Federal Forest Insect Research Program in the United States. The effects of mountain pine beetle are greatest in climax lodgepole pine forests where lodgepole pine is self-perpetuating or in even-aged stands where shade-tolerant species are not abundant enough to replace lodgepole pine. These even-aged forests are usually created by a stand-replacement fire. Mountain pine beetle infestations kill the largest trees and leave behind small-diameter, low-vigor, and mistletoe-infested trees. Young seedlings that have survived in the understory because of their shade tolerance are then released. New seedlings appear in new openings where trees have died. The resultant forest following mountain pine beetle outbreak is uneven aged and multistoried.

The life histories of tree-killing Dendroctonus (meaning "tree killers") species are generally similar. The female emerges from overwintering sites beneath the bark of trees killed in the previous year. She penetrates the outer bark and begins feeding in the phloem. She releases a pheromone component, trans-verbenol, and a host monoterpene hydrocarbon, myrcene (in lodgepole pine) or a-pinene (in western white pine). The male joins the female gallery and releases exo-brevicomin, another pheromone component. This mixture of compounds is highly attractive to other mountain pine beetles and a massive aggregation of beetles occurs on the tree. The attracted beetles introduce a pathogenic bluestain fungus, Ophiostoma clavigerum, which, together with tunneling females, causes the death of the tree. The growth of this bluestain fungus in the sapwood interrupts water conduction (a vascular wilt) to the crown. As the tree begins to die, females excavate egg galleries that are positioned vertically in the trunk, 30 to 90 cm in length. The egg gallery etches the sapwood superficially and may be quite sinuous, as with the western pine beetle, D. brevicomis. Eggs are laid in niches cut in the phloem on both sides of the gallery. Larvae hatch from the eggs and they excavate lateral galleries that increase in size as the larvae grow. A pupal cell is excavated at the end of the gallery, and the larvae transform to pupae and then to adults in these cells. Both larvae and adults are known to overwinter. Depending on the latitude and altitude one generation may take 2 years to complete or there may be as many as two generations and a partial third in 1 year.

Cryptococcus fagisuga (Beech Scale); Hemiptera: Eriococcidae (Eriococcids) The beech scale is a native European species that was introduced into Halifax, Nova Scotia, about 1890. Since then it has slowly spread eastward to Toronto, Ontario, and Ohio and south to West Virginia and western Virginia. Infestations are found in North Carolina, Tennessee, and Michigan. The beech scale in combination with the fungus Nectria coccinea var. faginata causes beech bark disease, which has killed a large number of native and ornamental beeches in Europe and the northeastern United States. European beech, Fagus sylvatica, and American beech, F. grandifolia, are severely impacted by this scale and its associated fungus, and it also infests other native as well as ornamental beeches from the Orient and Europe.

In late spring and early summer, pale-yellow females deposit up to 50 yellow-colored eggs in groups of 5 to 8 that are coated with a white-colored wax-like substance. Only wingless females are known for this species. The newly hatched nymphs or "crawlers" search out a location on the bark to settle and insert their tubular mouthparts. During this crawler stage they can be dispersed by wind, often over 100 meters. This sessile stage overwinters. After egg-laying the female dies. There is one generation produced each year. At high densities, the bark on the trunk and lower portion of the branches is completely white with scales. Under these conditions, the bark is killed and forms pits and ultimately ruptures, allowing entry of N. coccinea var. faginata. This fungus, along with the native species, N. galligena and N. ochroleuca, kills the cambium and sapwood in these areas, which results in interruption of water and food transport. The trunk often turns red with the fruit bodies produced by this fungus. Tree mortality often ensues after a few years. Extended periods of drought increase the rate of mortality caused by the interaction of this scale and fungus with their host. Cold temperatures (—38°C) often kill the scales above the snow pack. In Europe, the ambrosia beetle, Trypodendron domesticum, attacks beeches weakened by this scale and its associated fungus. Several decay-causing fungi are also associated with dying trees.

Sirex noctilio (Steely-Blue Wood Wasp); Hymenoptera: Siricidae (Horntails/Wood Wasps) S. noctilio is indigenous to Europe where it infests pines weakened by fire, insects, and diseases. In its native habitat it seldom causes tree mortality. However, this species was introduced into New Zealand in the late 19 th century, where it became a serious agent of mortality in planted forests of Monterey pine. It has since been introduced into Monterey pine plantations in Australia and South Africa. Females have a long ovipositor that extends straight back from the anus and is often mistaken for a stinger. The female drills a hole about 12 mm deep through the bark and into the sapwood. As many as four side holes are drilled from the primary entrance hole. One hole is filled with mucus from the female's accessory glands, as well as spores of the symbiotic fungus, Amylosterium areolatus, that reside in a specialized gland (mycangium) at the base of the

FIGURE 3 Female horntail, Urocerus gigas (Hymenoptera: Siricidae), ovipositing in the end grain of a European larch log. [Photograph by Andrew J. Storer.]

ovipositor. Eggs are deposited in the other side tunnels. Adults tend to aggregate on weakened trees for oviposition. The mucus secretion has toxic properties that cause interruption of water conduction in the sapwood (i.e., a vascular wilt). Needles are subsequently killed and they fall from the tree. Young larvae feed on the fungus growing in the egg chamber. Older larvae tunnel deeply into the sapwood and their finely divided frass (mostly woodchips) is tightly packed into their galleries. They pupate near the surface and excavate a round hole in the bark through which they emerge.

Horntails occur in both hardwood and softwood species and they are found in forests throughout the northern hemisphere (Fig. 3). The life cycle can be completed in 1 year, but in some cases development is delayed for several years.

A major biological control program for Sirex noctilio was undertaken and many species of hymenopteran parasitoids were collected from Europe and North America and introduced into New Zealand and Australia. Two species of nematodes, Deladenus siricidicola and D. wilsoni, were introduced into Australia where they have caused a significant reduction in tree mortality. These nematodes infect the reproductive system and prevent ovarian development.

Reticulitermes flavipes (Eastern Subterranean Termite); Isoptera: Rhinotermitidae Subterranean termites are social insects that live in colonies in the soil. They feed on a variety of cellulosic materials as their principal energy source. Termites and fungi are the most important organisms that can digest plant cell walls and thus reduce cellulose from a complex polysaccharide to simple sugars. Symbiotic fungi and protozoa live in the hindguts of termites and possess the enzymes needed to digest cellulose. Thus termites play an important role in recycling nutrients in a forest. Subterranean termites coexist in soils with many other microorganisms, especially fungi. Some of these fungi have been shown to produce chemicals that are the same as the trail-following pheromones produced by the termites, as well as chemicals that act as feeding stimulants and deterrents to termites.

The eastern subterranean termite is distributed throughout the eastern and mid-western part of the United States where it feeds on most species of wood found in forested habitats. Related species of subterranean termites are found in western North America and in Europe and Asia. Subterranean termite colonies contain seven castes: larvae or immatures, workers, soldiers, nymphs, winged (alate) primary reproductives, wingless (dealate) primary reproductives, and supplementary reproductives. Workers feed all the castes. Supplementary reproductives can occur in colonies with primary reproductives. They take over the oviposition role if the primary "queen" reproductive dies. Soldiers have large hardened (sclerotized) heads and mandibles, which they use to defend the colony from invaders, especially ants.

With the onset of warm rains in spring or fall (western United States), large numbers of winged females and males (termed swarmers) emerge from earthen tubes extending out of the ground or from the surface of logs, stumps, and wooden structures. These adults are weak fliers and soon drop to the ground where they lose their wings. Copulating adults paired end-to-end can be seen running about. The mated female seeks a damp piece of wood and excavates a chamber where she lays her first eggs. The young hatch into a worker caste and when large enough, they begin foraging for food to feed the queen and other castes as they are produced.

Subterranean termites are a critical component of forest habitats because of their important role in recycling wood. However, they are also the most destructive pests of human habitations made from wood. They construct earthen tubes from the ground into wooden structures and thus can consume wood in dry habitats long distances from the colony in the ground. They often escape notice because they travel in hollowed out timbers and in tubes constructed in wall cavities. Their galleries parallel the grain of the timbers and are excavated first in the early wood (springwood) and later in the late wood (summerwood). This damage to structures can be extensive before it is discovered. A large pest control industry has developed to protect wooden structures from termites and other wood-destroying insects and fungi.

Synanthedon sequoiae (Sequoia Pitch Moth); Lepidoptera: Sesiidae (Clearwing Moths) The sequoia pitch moth is attracted to the resin that flows from wounded pine trees. Larvae hatch from eggs deposited individually on the bark of limbs and the trunk and tunnel into the phloem—cambial layer beneath the outer bark. As they feed on the phloem, they excavate a chamber that fills with resin and frass. This material is deposited on the outer bark surface over the chamber and forms a white resinous mass that is interspersed with reddish brown fecal material. As this resin mass ages, the surface turns reddish brown. Resin streaming is occasionally observed from these masses. Larvae pupate beneath the resinous mass, but with the pupal integument extending through the mass. This enables the moth to emerge from the mass without contacting the resin. These resinous masses often exhibit two or three pupal cases, indicating that these areas are often recolonized by later generations of this species. Oviposition occurs in summer. The life cycle is completed in 1 or 2 years.

Infestations of this species rarely cause tree mortality directly. However, heavy infestations may result in attraction of tree-killing bark beetles. This moth infests many native species of pines as well as European species such as Scots pine, Austrian pine, Pinus nigra, maritime pine, P. pinaster, and Italian stone pine, P. pinea. Pines growing in urban environments can be heavily infested around wounds caused by pruning and by vehicles. The sequoia pitch moth is distributed throughout western North America, including California, Oregon, Washington, Idaho, Montana, and British Columbia. Coast redwood, Sequoia sempervirens, is not a known host for this moth. Similar pitch masses are produced by a related moth, S. pini, which infests many pine and spruce species in eastern North America and in the midwestern states.

Insects That Feed on Roots

Insects that feed on the roots of trees may be sap feeders or may enter the host tissue and exploit the phloem, cambium and/or xylem. Representatives are found in the Coleoptera, Hemiptera, and Hymenoptera. The underground portion of the tree represents a protected environment for root-feeding insects.

H. abietis (Pine Weevil); Coleoptera: Curculionidae

(Weevils) The pine weevil is native to the forests of northern Europe and Asia where it colonizes roots of weakened and recently dead conifers. Larvae emerge from eggs laid in the bark and tunnel through the phloem and score the sapwood surface. They pupate in cells excavated in the outer sapwood. Its abundance has been increased dramatically by thinning and clear-cutting of large forested areas and replanting with pines. Under these conditions, the weevils breed in the stumps and emerge in the next season to feed on seedlings planted after logging. This species has the reputation of being one of only a few species of which one individual can kill one or more healthy trees, although they are small trees. Grasshoppers (Acrididae) and scarabs (Scarabaeidae) also have this capability. Adults feed on the tender young bark of most coniferous species. They are present from early spring through early fall. Control of this species is necessary in new conifer plantations. One generation is produced in 1 to 3 years, depending on latitude. The pales weevil, H. pales, has a comparable biology in the eastern United States. These insects are considered to be among the most destructive pests of conifer plantations in both Europe and North America.

Pachypappa termulae (Spruce Root Aphid); Homoptera: Pemphigidae (Pemphigids) The spruce root aphid is cream colored and feeds in colonies on the fine roots of standing trees. These colonies are covered with waxy wool. Spruce trees are most commonly infested, though other conifers may also be affected. As with all aphids, this is a sapfeeding species that has sucking mouthparts. Aboveground symptoms are usually absent except in nursery stock, where an overall decline of plant health may be observed. Often these insects are noticed only during transplantation of nursery stock. This species can be found year round in parts of Europe and North America and is thought to be associated with dry conditions.

Camponotus modoc (Carpenter ant); Hymenoptera: Formicidae (Ants) C. modoc excavates galleries in fire-scarred and rotted standing and fallen trees of many coniferous species growing in western North America, including pines, true firs, Douglas-fir, western redcedar, Thuja plicata, and giant sequoia. In a study of uprooting and breakage of over mature giant sequoia in the Sierra Nevada of California, carpenter ants and decay fungi were often associated with tree failure. Most of the roots of recently uprooted trees showed evidence of advanced decay. Carpenter ant galleries were observed in the few functional roots that were present. These roots showed evidence of early-to-moderate stage decay and carpenter ants were observed tunneling in these areas. This excavation activity is probably associated with the establishment of subsidiary colonies. These ants are found tending aphids, e.g., Cinara spp., in colonies on understory white fir and sugar pine trees. C. modoc was also observed excavating cavities in the bark of the root collar of young white firs. These cavities were later colonized by Cinara spp. that were tended by these ants. These shade-tolerant conifers have greatly increased in abundance because of fire exclusion in the old-growth giant sequoia groves. Thus carpenter ant abundance may have also increased as a result of increased aphid colonies on these understory trees.

Many insect families are found almost exclusively in forest habitats and have not been mentioned above because of space limitations. These include, for example, the wood-boring beetles in the families Buprestidae, Cerambycidae, Platypo-didae, Micromalthidae, Anobiidae, Bostrichidae, and Lyctidae. In the Lepidoptera, the family Cossidae is almost entirely found infesting wood. Similarly, a very large number of species of moths, wasps, aphids, and leafhoppers feed on the foliage of trees.

0 0

Post a comment