Lethal and sublethal effects of preimaginal treatments with two pyrethroids on the life history of the egg parasitoid Telenomus busseolae

Abstract

The egg parasitoid Telenomus busseolae Gahan (Hymenoptera: Scelionidae) is one of the most important natural enemies of the Mediterranean corn stem borer, Sesamia nonagrioides Lefebvre (Lepidoptera: Noctuidae). The effects of lethal and sublethal concentrations of two commonly used pyrethroids, deltamethrin and cyfluthrin, on emergence, fecundity, longevity, and offspring sex ratio of T. busseolae exposed during different preimaginal stages were studied. The insecticide, the tested concentrations of the insecticide (field-recommended concentration and three dilutions with twofold concentration decreases), and the stage of parasitoid preimaginal development (first and second larval stages and pupae) all significantly influenced the percentage of emergence. Life history parameters of parasitoids exposed to the highest and lowest tested concentrations of the two insecticides (3.0 and 0.375 ml l−1 for cyfluthrin and 0.5 and 0.0625 ml l−1 for deltamethrin) at eight days post-oviposition (pupal stage) were calculated. The intrinsic rate of increase, the total progeny produced, and the net reproductive rate of the parasitoid were all adversely affected by the field-recommended concentrations of at least one insecticide. However, the longevity, the length of the oviposition period, and the offspring sex ratio were unaltered by the two insecticides. The effects of sublethal concentrations of the insecticides, applied at preimaginal stages, on the responses of T. busseolae females to host abdominal scales, were studied in open-arena bioassays. Cyfluthrin altered both the residence time and linear speed of the parasitoid in the host scale-containing areas, whereas deltamethrin had no effect. Our results indicate that, in addition to lethal effects, sublethal effects should also be considered when evaluating insecticide consequences on non-targets.