Reproductive strategy of a woodwasp with no fungal symbionts, Xeris spectrum (Hymenoptera: Siricidae)

Abstract

Experiments were conducted to elucidate the reproductive strategy of the siricid woodwasp, Xeris spectrum, which carries no substantial symbiotic fungi in its body, in a comparison with the life cycles of two fungus-carrying siricid woodwasp species, Sirex nitobei and Urocerus japonicus, by considering ecological traits such as seasonal patterns of occurrence, spatial distribution of emergence on a tree, and oviposition activities. Part of the X. spectrum populations emerged in spring, during May and June, while others emerged in summer, during August and September, simultaneously with other siricid fungus-carrying woodwasps. The vertical distribution pattern of X. spectrum emergence holes on the trunk closely coincided with the emergence hole pattern of S. nitobei. X. spectrum laid few eggs on fresh logs, old logs, or on logs inoculated with potato dextrose agar, whereas on logs inoculated with Amylostereum chailletii or A. areolatum, X. spectrum females oviposited no less than 30%, on average, of their potential eggs. Moreover, the oviposition sites on these logs were concentrated near the Amylostereum inoculation positions. These results indicate that X. spectrum has evolved a life history that utilizes fungal symbionts of other woodwasp species without itself possessing any symbiotic fungus. Moreover, X. spectrum has evolved a dual reproductive system in that (1) some adults emerge in summer during the same emergence period as the fungus-carrying woodwasps and thereby oviposit on host trees already inoculated with fungi, and (2) other adults emerge the next spring and oviposit on trees that were inoculated with fungi a year earlier.