Egg and Larval Development in the Woodwasp, Sirex Noctilio F.

Abstract

Development and survival of the immature stages of Sirex noctilio F. are dependent on active growth of the symbiotic fungus Amylostereum areolatum. In laboratory and field studies in Tasmania, suppression of fungal growth by excess moisture or temperature prolonged embryogenesis so that eclosion might be delayed until 300 days after oviposition in the base of some attacked trees (Pinus radiata), as compared with 15-30 days in warmer and drier tops of the same trees. Enhanced respiratory activity of the fungus might trigger egg development, for eggs exposed to carbon dioxide matured more rapidly than those exposed to air. Pure oxygen and nitrogen were not toxic to the eggs, and surface treatment with dilute ethanol delayed development. The development of the egg of Sirex is described for the first time. The larvae fed on fungus-infested wood; the conversion ratio of larval weight change to weight of wood displaced per instar decreased with successive instars. This reduction resulted from the progressive decline of fungal food substrate and overall moisture content. The ultimate size of adults of S. noctilio was determined by conditions that favoured fungal growth, and the fungus transported nitrogen through the wood to the feeding larvae.