Effects of parasitoid strain and host instar on the interaction of Bacillus thuringiensis subsp. kurstaki with the gypsy moth (Lepidoptera: Lymantriidae) larval parasitoid Cotesia melanoscela (Hymenoptera: Braconidae)

Field studies suggest a positive relationship between application of the microbial pesticide Bacillus thuringiensis Berliner subsp, kurstaki and use of the larval parasitoid Cotesia melanoscela (Ratzburg) against the gypsy moth Lymantria dispar (L.). Under controlled conditions, however, exposure of gypsy moth larvae to sublethal concentrations had effects on C. melanoscela that varied with parasitoid strain, parasitoid life stage, and host instar. Performance of French-strain wasps emerging from hosts exposed as 2nd-instars was enhanced by host ingestion of B. thuringiensis subsp. kurstaki. Conversely, Korean-strain wasps showed lesser and inconsistent effects. Enhanced parasitoid performance was attributed to increased larval, as opposed to adult, emergence. In 2nd-instar hosts, parasitoid survival decreased between larval and adult emergence. Experiments with 3rd-instar gypsy moths showed negative effects on C. melanoscela. Although the mechanisms for these differences are not clear, these results suggest that judicious timing and strain selection can enhance integration of microbial pesticides and parasitoids.