Influence of insecticide application on host finding of the cabbage stem weevil parasitoid Tersilochus obscurator (Hym.; Ichneumonidae)

Abstract: Host location of hymenopterous parasitoids is mainly based on olfactory cues emitted from the infested host plant or host larvae. In this study, we used behavioural bioassays to determine sub-lethal effects of insecticide residues on host finding of Tersilochus obscurator (Hym.; Ichneumonidae), a specialist parasitoid of the larvae of cabbage stem weevil, Ceutorhynchus pallidactylus (Mrsh.) that feeds within petioles and stems of oilseed rape. In Y-tube olfactometer experiments, T. obscurator females significantly preferred volatiles emitted from untreated leaves to volatiles emitted from leaves treated with the neonicotinoid insecticide thiacloprid, but not to volatiles emitted from leaves treated with the pyrethroid lambda-cyhalothrin. In dual-choice experiments females spent less time foraging on insecticide-treated leaves compared to untreated leaves or even avoided treated leaves. Further, on insecticide-treated leaves they performed less ovipositor probes than on untreated leaves. Thus, sub-lethal effects of insecticides may substantially reduce the level of parasitism of oilseed rape pests in the field.