Resource utilization and reproductive strategy of three woodwasp species (Hymenoptera : Siricidae)

Woodwasps (Siricidae) are hymenopterans whose larvae feed primarily on the sapwood of various coniferous and broad-leaved trees. In many species of woodwasp, the females carry arthrospores of one of the specific basidiomycetous fungi, Amylostereum spp., as a symbiont in a pair of small intersegmental sacs in their bodies. Females drill several holes through the bark deep into the sapwood of new host trees and then deposit arthrospores of the fungus together with eggs in the holes. The mycelia propagated in the wood tissues or fungus-infected parts of the wood are likely to provide essential nutriments and/or digestive enzymes which decompose wood tissues for the larvae. Recently in Japan, woodwasps have been regarded as a forest pest, because the discoloration and deterioration of wood tissue induced by the symbiotic fungi have often degraded the commercial value of the wood. During a six-year period of study, I investigated ecological traits such as life history and oviposition system, the symbiotic relationship between woodwasps and fungi, and the interactions between organisms involved in the system, for three woodwasp species, Sirex nitobei, Urocerus japonicus and Xeris spectrum which infested Pinus densiflora, Cryptomeria japonica, Chamaecyparis obtusa and/or Abies firma. The results are summarized as follows. (I) Ecological traits of the three woodwasp species 1. Three genera and three species of woodwasp, S. nitobei, U. japonicus and X. spectrum, emerged from the four host tree species. The life cycle of S. nitobei was completed in one year. Most of S. nitobei emerged from P. densiflora or A. firma from early September to mid-October. The life cycle of U. japonicus was also completed in one year, and most emerged from C. japonica or C. obtusa from early July to mid-September. On the other hand, X. spectrum had two types of population, a population emerging from C. japonica or A. firma from mid-August to late August one year after oviposition, and another population emerging from these host-trees during early May to mid-June two years after oviposition. 2. The body weight ranged from 20-490 (♀) and 11 -226 mg f. wt (♂) in S. nitobei, 42-299 (♀) and 17-84 mg f. wt (♂) in U. japonicus and 12-181 (♀) and 3-62 mg f. wt (♂) in X. spectrum. The values varied greatly among individuals in all the woodwasp species. The mean number of eggs produced per female (potential fecundity) was largest in U. japonicus (370), followed by S. nitobei (150) and least in X. spectrum (130). On the other hand, the egg size evaluated by egg length was largest in S. nitobei (1.6 mm), followed by X. spectrum (1.4 mm) and least in U. japonicus (1.1 mm). The mean longevities of the three species ranged from 3.5-5.5 days. (2) Fecundities of the three woodwasp species and host- tree conditions affecting their oviposition behavior 1. The potential fecundities of the three woodwasp species were approximately proportional to the fresh body weights of female adults at the time of emergence, and this regression allowed estimates of the number of eggs laid and the proportion of oviposition. Using these relationships, the oviposition traits of the three species on their major host trees were compared using fresh logs within one week after tree-felling. The mean proportion of oviposition was estimated to be about 54% (range 0-100%) on P. densiflora logs by S. nitobei and 79% (50-100%) on C. japonica logs U. japonicus, in marked contrast to only 3% on C. japonica logs by X. spectrum. 2. According to the days elapsed after tree-felling, the oviposition proportions of S. nitobei on P. densiflora logs can be divided arbitrarily into three groups. The oviposition proportions on the logs within 0-3 days after tree-felling ("fresh logs") varied greatly from log to log, and about half of female adults showed proportions less than 50% (mean: 39%). However, all the females that oviposited on the logs 4-24 days after tree-felling ("intermediate logs") invariably showed p