Minimizing the disruptive effect of Ips grandicollis (Coleoptera: Scolytinae) on biocontrol of Sirex noctilio (Hymenoptera: Siricidae)

- Sirex noctilio biocontrol has been declining in recent years in Australia.
- Ips grandicollis attacks trap trees prepared for S. noctilio biocontrol.
- Dose of herbicide used to weaken trap trees and optimal timing were investigated.
- Treating trap trees in mid- summer best promotes S. noctilio biocontrol.

Sirex noctilio Fabricius (Hymenoptera: Siricidae) is an invasive wood wasp that can cause significant tree mortality to Pinus. Trees treated with herbicide are used to attract S. noctilio to oviposit and are the means by which the nematode biocontrol agent, Deladenus (=Beddingia) siricidicola Bedding (Nematoda: Neotylenchidae), is introduced into the pest population. In Australia, these trap trees are also attacked by the invasive bark beetle, Ips grandicollis Eichhoff (Coleoptera: Curculionidae; Scolytinae). This study investigated the effect of herbicide dose and timing of application on subsequent attractiveness of trap trees to S. noctilio and I. grandicollis. Two replicated trials were conducted concurrently in two major pine growing regions of Australia. Trees in southern New South Wales and South Australia were treated in a factorial manner with two doses of glyphosate on two dates (December and January) along with an untreated control. In both locations, S. noctilio attacked January-treated trees sooner after herbicide application compared to December treatments. Ips grandicollis attacked December-treated trees sooner compared to January treatments. January treated trees were best in attracting S. noctilio before I. grandicollis attack-irrespective of herbicide dose-in South Australia, but not in New South Wales. Numbers of emerging S. noctilio and those parasitized by nematodes were negatively correlated with the number of days trap trees were infested by I. grandicollis. In New South Wales, herbicide application to trap trees in January (mid-summer) best promotes S. noctilio biocontrol by minimizing colonization by I. grandicollis, but this was not as clear-cut in South Australia.

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