A chemical study of the bearing of decay by Phellinus cryptarum Karst. and other fungi on the destruction of wood by the death-watch beetle (Xestobium rufovillosum de G.)

Abstract

The natural habitat of Xestobium is in dead and decayed parts of hardwood trees. Fisher has shown that as the severity of fungal decay in wood increases the duration of the life cycle of X. becomes less. The bearing of fungal decay on the availability of oak wood to X. has now been examined from the chemical standpoint. Matched samples of oak (Quercus sp.) sapwood were exposed to decay by P. cryptarum in pure culture and classified according to % loss in wt. sustained during decay. The fungus is responsible for a white rot in which all the major wood components are decomposed simultaneously in approx. the same proportions in which they occur in the original wood. Samples of decayed wood of known composition were exposed to attack by X. under standard conditions and analyzed thereafter. Analyses are given of the material digested by the larvae. The diet consists of extractives, cellulose, hemicelluloses and lignin. The wt. of wood digested per 100 gm. of wood disintegrated during boring by is termed the "abstraction co-efficient" (a.c). The numerical value of the a. c. declines as the extent of previous fungal decay in the substrate increases, showing that the prime function of the fungus in this biological succession is not predigestion of wood components. Depreciation of the mechanical strength of wood induced by fungal decay is the major factor responsible for the more favorable development of the larva in decayed as opposed to less decayed wood. Decayed wood offers less resistance to larval boring than sound wood. During progressive decay the rate of decline in the strength of wood is greater than the rate of decline in density. It is also possible that the rate of decline in the larval a. c. is less than the rate of decline in the strength of the substrate. These factors offer advantages to the X. larva so far as its energy metabolism is concerned. The rate of larval boring increases as the extent of fungal decay in the substrate increases. In its natural habitat the X. larva may therefore regulate its rate of boring so that as the mechanical strength of the substrate decreases it is enabled to increase its rate of abstraction and conservation of energy. Several cases of X. attack in structural timbers taken from buildings in England were examined. Such timbers are usually more or less decayed and in the majority of cases by brown-rot fungi. No specific fungus is associated with X. and insect attack is not limited to wood which has attained any particular stage of decay.