Incidence of Ips grandicollis (Coleoptera: Scolytinae) in trap trees prepared for biological control of Sirex noctilio (Hymenoptera: Siricidae) in Australia: Influence of environment and silviculture

In Australia, the bark beetle Ips grandicollis (Eichhoff) (Coleoptera: Curculionidae) attacks trap trees prepared for Sirex noctilio Fabricius (Hymenoptera: Siricidae) biocontrol, potentially threatening biocontrol programs for this invasive pest. Trap trees are prepared in the summer to attract S. noctilio females to oviposit for later introduction of the nematode Beddingia siricidicola (Bedding), which is the biological control agent that has successfully managed populations of this exotic pest. In Australia, the bark beetle I. grandicollis has unprecedentedly been attacking these trap trees and the magnitude of the threat facing S. noctilio biocontrol as a result of attack by the bark beetle is unknown. Surveys for incidence of I. grandicollis in trap trees were conducted in four states of Australia - New South Wales, South Australia, Tasmania and Victoria - where P. radiata is grown commercially and S. noctilio biocontrol is important. Results showed that I. grandicollis attack of trap trees is widespread and prevalent in South Australia, southern New South Wales and Victoria and absent in Tasmania.The incidence of I. grandicollis in the trap trees was more severe in South Australia compared with New South Wales and Victoria. A more detailed analysis using New South Wales data examined the relative importance of biotic and abiotic factors in determining attack of trap trees by I. grandicollis in order to explain and allow prediction of the patterns of attack by the bark beetle and potential threat to S. noctilio biocontrol. The whole model accounted for 58.7% of the deviance; with the key predictors accounting for 46.3%. A linear regression model showed that maximum summer and winter temperatures, lower (0.5-1.9. m) soil moisture two years before the surveys, summer upper (0-0.7. m) soil moisture in the year of the surveys, autumn lower soil moisture in the year of survey, age of trap trees and damage to trees adjacent to trap tree plots are key predictors of I. grandicollis attack on trap trees. Some of these driving factors were expected; others were unexpected or contradictory to our expectations. These factors would be important considerations when identifying locations where trap tree plots are established to reduce the impact of I. grandicollis on the S. noctilio biocontrol program.