Evaluation of the insect parasitoids of Sirex noctilio (Hymenoptera: Siricidae) in Tasmania.

Sampling of Sirex noctilio F. and the insect parasitoids released in a forest of Pinus radiata D. Don near Hobart, Tasmania, has shown that the rhyssines, Megarhyssa nortoni (Cresson) and Rhyssa persuasoria (L.) were mainly responsible for reducing the level of the population of S. noctilio between 1965 and 1974. Parasitic nematodes were not present during this period. Key factor analysis indicates that in this forest the two species act as one delayed density-dependent factor, whereas Ibalia leucospoides Hochenw. is apparently density-independent, its ability to parasitize the host being regulated by conditions within the tree.