Insect-Wood Decay Complexes
Of all the pests that reduce hardwood lumber production, none are more important than the wood decay fungi and the hardwood borers. Those capable of acting together in symbiotic complexes are even more damaging. Recent U.S. Forest Service research at the Southern Hardwoods Laboratory in Stoneville, MS, has been aimed at identifying and quantifying losses caused by important insect and disease pests that are causing substantial reductions in hardwood lumber production and value. This work has revealed new, previously unknown woodwasp-wood decay fungi complexes capable, in some cases, of causing considerable damage to logs in standing trees, ultimately reducing hardwood lumber value. These wood decay fungi are mycosymbionts of a peculiar group of insects, the woodwasps (Hymenoptera: Siricoidea), with larval stages that bore through the wood of stressed and weakened hardwood trees and cause significant damage by forming galleries and vectoring (transmitting) wood decay fungi in the process (Gilbertson 1984, Smith 1979).
Like most wood-feeding insects, woodwasps must live in symbiotic relationships with wood decaying microbes because they are incapable of digesting cellulose. The decay fungi are carried in special glands (mycangia) at the base of the abdomen near the ovipositor in female woodwasps. The adult female woodwasp stores inoculum of the wood decay fungus in these mycangial glands, which are connected directly to the oviduct that passes through the ovipositor. The decay fungus is injected into the wood with the eggs during oviposition. The fungus then grows rapidly and produces extracellular cellulases, which digest the wood for larval consumption (Kukor and Martin 1983). When the eggs hatch, the larvae begin boring through the decayed wood, consuming nutrients both from the decayed wood and the mycelium of the fungus itself. The larvae cannot consume and digest the wood until it is decayed by the enzymes of the fungus. The larvae produce extensive galleries throughout the rotting wood, eventually pupate in the wood, and emerge as adults making round exit holes. The wood is decayed far beyond these borer galleries in all directions. Most of these fungi grow very rapidly through the wood, and the wood is decayed almost completely over several years as both the cellulose (wood fibers) and lignin are digested by extracellular enzymes (Wilson and Schiff 2003). Thus, all of these fungi are physiological white rotters. The actions of these two pests together result in synergistic damage to and economic loss of merchantable hardwood lumber volume. The decay fungi also produce discoloration in the wood (a type of stain called zone lines) that further degrade lumber value. The zone lines, produced within decaying wood in association with these wood decay fungi complexes, are a result of somatic antagonism (SA) between different strains of the wood decay fungi competing for the same wood substrate (Wilson and Schiff 2000a). Zone lines that form in wood as SA interactions between xylariaceous fungi represent areas delimiting their territory around decay zones (see figure at right). The wood becomes riddled with all three types of damage (borer galleries, decay, and discoloration of wood) until the entire branch or bole becomes unmerchantable. This is a perennial process in which the damage may be compounded by repeated infestations of branches and boles by subsequent generations of the woodwasp.
Two major groups of woodwasps can affect hardwood lumber production. The large, siricid woodwasps (Siricidae: subfamily Tremicinae) attack predominantly oaks, sugarberry (Celtis laevigata Willd.), beech (Fagus grandifolia Ehrh.), and other bottomland hardwood species. Smith and Schiff (2002) provide a review and keys to the siricid woodwasps of the Eastern United States. These tremecine woodwasps vector predominantly basidiomycetous wood decay fungi. The two most common species are Tremex columba (Linnaeus) and Eriotremex formosanus (Matsumura). The smaller xiphydriid woodwasps attack mostly maples, elms, and upland hardwood species. They carry ascomycetous wood decay fungi that form spores in microscopic sacks (asci) inside of perithecia embedded in black stromatic tissues that develop on the surface of the wood. The woodwasp family (Xiphydriidae) has 22 described genera, approximately 100 species, and a worldwide distribution (Smith 1978). The family is represented in the United States by a single genus, Xiphydria, with 10 described native species.
Xiphydriid woodwasps oviposit primarily into the axils of living hardwood branches, causing extensive decay and galleries in this area. This eventually weakens the limb which may then be broken by wind or ice accumulation. The decay can extend into the sapwood of the bole through the remaining branch stub after the limb falls off. Xiphydriid larvae continue to bore and develop in the fallen limb on the ground. Larvae produce galleries throughout the wood until the following spring, pupate, and emerge as adults (Solomon 1995). Adults mate (optionally) and oviposit their eggs once again into the axils of living branches or into dead limbs on the ground to complete the cycle. Most woodwasp species seem to be fairly host-specific, often attacking only one or two hardwood species, although a few species such as X. tibialis have a number of hardwood hosts. There also appears to be high fidelity in the symbiotic association between woodwasp species and their fungal symbiont.
Woodwasp-wood decay fungi complexes have been found in every major hardwood species. Thus, these pest complexes are potentially significant sources of log defects in all hardwood stands. The occurrence of a new, nonnative siricid woodwasp [Eriotremex formosanus (Matsumura)] in hardwood forests of the Southern United States is of considerable concern because this pest has spread from Georgia to Texas since its introduction into the United States in infested wooden shipping crates brought back by the military from Southeast Asia after the Vietnam War in the early 1970s (Smith 1996). Recent decay tests in vitro have demonstrated that the wood decay fungus vectored by this woodwasp has the potential to rapidly decay sapwood in many eastern hardwoods (Wilson and Schiff 2003). This fungus does not fruit readily on its oak hosts or in vitro. This makes identification difficult because the teleomorph or sexual stage contains key taxonomic characters required for identification. Perhaps these symbiotic fungi do not normally produce sexual fruiting bodies because they are regularly carried to appropriate tree hosts by their woodwasp vector and, therefore, do not have to expend energy to produce a metabolically costly fruiting body for sporulation and wind dispersal in order to survive. This is why the extensive damage caused by these pests often goes unnoticed until the tree is cut. These wood decay fungi are rarely visible on the outer surface of trees, and adult emergence holes of the woodwasps look similar to those of other hardwood borers. Consequently, the extent to which these pests are damaging southern oak forests is not known, although preliminary results with wood decay studies in vitro indicate that the mycosymbiont of E. formosanus and those of xiphydriid woodwasps can cause substantial white rots in eastern hardwoods after only 1 year (Wilson and Schiff 2003). We do not yet understand the importance of the role woodwasps play in the dispersal of wood decay fungi, the impact they have on forest health, or the effects that nonnative pests such as E. formosanus and its symbiont will have on lumber defect losses, hardwood timber salvage, and forest decomposition cycles. Further research is needed to elucidate the roles played by these new pest complexes that are invading our southern forests so that appropriate control strategies can be developed. These insect-disease pest complexes will likely receive increasing attention in the future as forest managers become more aware of their existence, their potential to cause damage, and their long-term impact on lumber production in hardwood forests.
Encyclopedia ID: p985
