Interactions between the pine pests Sirex noctilio (Hymenoptera: Siricidae) and Ips grandicollis (Coleoptera: Scolytidae) and their associated fungi and nematodes

The woodwasp, Sirex noctilio F., is a major pest of introduced pine plantations in
Australia. A parasitic nematode, Deladenus (= Beddingia) siricidicola (Bedding) has been used to effectively manage S. noctilio in Australia. A free-living form of the nematode feeds and reproduces on the symbiotic fungus of S. noctilio, Amylostereum areolatum (Chaillet ex Fr.) Boidin whilst in the wood of trees prior to it parasitising S. noctilio larvae. The nematode is introduced into forests by inoculating ‘trap trees’ that had previously been treated with herbicide to make them attractive oviposition sites for S. noctilio. Nematode parasitism does not kill S. noctilio but sterilises adults such that females lay ‘packets’ of nematodes rather than eggs and this effect is important in the spatial and temporal distribution of the agent in the host population. This historically successful biocontrol program is seriously threatened by the recent, unprecedented
increase of another exotic pest, bark beetle, Ips grandicollis (Eichhoff) which attacks trap trees. This situation is exacerbated by recent periods of prolonged drought in Australia which increase stress on forests making them more vulnerable to pest attack.

This study investigated mechanisms that could be responsible for causing
biocontrol disruption. Experimentally imposed elevated temperature resulted in smaller S. noctilio and disrupted egg development and maturation. The rate of nematode infection of normal eggs in parasitised S. noctilio decreased with an increase in temperature. If this laboratory result also occurred in the field it would compromise the dispersal of the nematodes. Elevated temperature also adversely affected egg development of S. noctilio so, irrespective of any effect on biocontrol, adaptation to climate change and range expansion into subtropical pine plantations may not be possible for this pest.

In the second phase of this study, the direct and indirect effect of I. grandicollis
infestation and interactions of its microbial associates with S. noctilio and D.
siricidicola within the trap trees were determined by field and laboratory experiments. Ips grandicollis attack led to accelerated wood drying and affected the size of emerging female S. noctilio. Reductions in tree moisture content lowered nematode parasitism of both S. noctilio sexes. The observed effects of I. grandicollis on S. noctilio are detrimental to the inoculative biocontrol program which requires high emergence of large, parasitised female S. noctilio to disperse the nematode agent in the wild population.

Laboratory experiments were conducted to investigate whether biocontrol
disruption involves the I. grandicollis-associated fungus, Ophiostoma ips (Rumbold) Nannf.,. Ophiostoma ips competitively excluded A. areolatum and contributed to tree dryness which also affected the growth of A. areolatum. Deladenus siricidicola failed to survive and reproduce on O. ips as it can on A. areolatum. Higher temperatures favoured the growth of O. ips and negatively affected growth of A. areolatum and D. siricidicola. Competitive interactions between O. ips and A. areolatum within trap trees are demonstrated to be a key factor in the negative effect of I. grandicollis on the S. noctilio biological control program.

Overall, this thesis demonstrated that attack by I. grandicollis is adversely
affecting the S. noctilio biocontrol program and has elucidated the mechanisms by which this is occurring. More generally, the findings signal the need for further work to develop management strategies for this multi-pest, multi-microbial associate system as well as to better understand the changes that may result from climate change.