Studies of Sirex noctilio and its natural enemies, in herbicide-treated trap trees of radiata pine in Victoria.  F. G. Neumann, S. J. Smith and J. L. Morey.  October 1984.  16 pp. (unpubl.)

SUMMARY

From 1979 to 1982 in North-eastern Victoria, interactions between the European tree-killing woodwasp Sirex noctilio Fabricius (Hymenoptera:Siricidae),the parasitic nematode Deladenus siricidicola Bedding (Neotylenchidae), and the wasp parasitoids Ibalia leucospoides Hochenwarth (Ibaliidae) and Megarhyssa nortoni nortoni Cresson (Ichneumonidae) were studied within the sapwood of 20% dicamba herbicide-treated trap trees of Pinus radiata D. Don (radiata pine). The level of biological control over time in such trees was also assessed.

Seventy 1.0 m long Sirex-infested billets, randomly cut from a total of 162 trap trees, were examined in detail; almost 700 parasitoid emergents were tallied by species and sex; and the ovaries or testes of approximately 2000 S. noctilio emergents were dissected for nematode infection.

Populations of the nematode did not affect parasitoid activity or survival, nor restrict fungal food supply to S. noctilio immatures. Of the three biological control agents studied, 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 33.8% mortality of S. noctilio immatures, in the second generation of trap trees. Among the four species of parasitoid (Rhyssa persuasoria L. and R. hoferi Rohwer (Ichneumonidae) were also released) liberated since 1976 near Myrtleford, only I. leucospoides has shown potential as a control agent.

Thus, biological control of S. noctilio in North-eastern Victoria predominantly involves the suppression of the pest's reproductive capacity by the parasitic nematode D. siricidicola. This means that a system of strategically placed, herbicide-injected trap trees needs to be maintained for a minimum two-year period, along with established populations of D. siricidicola and I. leucospoides,

to achieve rapid and effective biological control of S. noctilio in highly susceptible unthinned plantations of intermediate age.

Also published:

Neumann, F.G. and Morey, J.L. (1984)  Influence of natural enemies on the sirex woodwasp in herbicide-treated trap trees of radiata pine in North-eastern Victoria.  Aust. For. 47 (4): 218-24.