Influence of natural enemies on the Sirex wood wasp in herbicide-treated trap trees of radiata pine in north-eastern Victoria

Summary

From 1979 to 1982 in north-eastern Victoria, interactions between the European tree-killing wood wasp Sirex noctilio, the parasitic nematode Deladenus siricidicola, and the parasitoids Ibalia leucospoides and Megarhyssa nortoni nortoni were studied within the sapwood of Pinus radiata trap trees treated with 20% dicamba herbicide. The level of biological control over time in such trees was assessed.

Populations of the nematode did not affect the activity or survival of the parasitoids, nor restrict the fungal food supply to immature S. noctilio. Of the three biological control agents, the nematode was the most effective, inducing almost 100% sterility in adult S. noctilio females (and a similar infection level among males), whereas parasitoids altogether caused only a maximum of 34% mortality of immature S. noctilio, in the second generation of trap trees. Among the four species of parasitoid released since 1976 near Myrtleford (Rhyssa persuasoria and R. hoferi were also liberated), only 1. leucospoides showed any promise as an agent capable of contributing to Sirex control.

Thus, biological control of S. noctilio in north-eastern Victoria mainly depends on the suppression of the pest's reproductive capacity by the parasitic nematode D. siricidicola. In highly susceptible unthinned plantations of intermediate age, this can best be achieved by maintaining for a minimum of two years a system of strategically placed trap trees injected with herbicide, along with populations of D. siricidicola and I. leucospoides.