A major forest insect pest, the pine weevil Hylobius abietis, is more susceptible to Diptera- than Coleoptera-targeted Bacillus thuringiensis strains.

BACKGROUND The pine weevil (Hylobius abietis) is a major forest regeneration pest causing high levels of seedling mortality and economic losses. Current management relies on silviculture, stem coatings, and insecticides. Here, we evaluated for the first time, the effects of Bacillus thuringiensis (Bt) strains on H. abietis adults: two, producing the Coleoptera-targeted toxins Cry3Aa (Bt tenebrionis NB-176) and Cry8Da (Bt galleriae SDS-502), and one producing the Diptera-targeted Cry10A (Bt israelensis AM65-52). Choice and non-choice assays using individual and mixtures of Bt formulations, containing these strains respectively, were conducted. RESULTS We found that Bt had toxic and lethal effects on H. abietis, but effects varied with strain and formulation concentration. The Diptera-targeted Bt israelensis had the most negative effects on weevil weight, feeding and mortality (70-82% feeding reduction, 65-82% greater mortality than control), whereas the effect was lower for the Coleoptera-specific Bt tenebrionis (38-42%; 37-42%) and Bt galleriae (11-30%; 15-32%). Reduced weevil feeding was observed after 3 days, and the highest mortality occurred 7-14 days following Bt exposure. However, we found no synergistic toxic effects, and no formulation combination was better than Bt israelensis alone at reducing consumption and survival. Also, pine weevils were not deterred by Bt, feeding equally on Bt-treated and non-Bt treated food. CONCLUSION There is potential to develop forest pest management measures against H. abietis that include Bt, but only the Diptera-targeted Bt israelensis would provide effective seedling protection. Its Diptera-specificity may need reconsideration, and evaluation of other Bt strains/toxins against H. abietis would be of interest.