Side effects of two reduced-risk insecticides, indoxacarb and spinosad, on two species of Trichogramma (Hymenoptera: Trichogrammatidae) on cabbage

Trichogramma pretiosum Riley and T. brassicae Bezdenko are common egg parasitoids of many lepidopteran pest species damaging vegetable, but their effectiveness can be severely curtailed by insecticide applications. To identify insecticides that are potentially compatible with these parasitoid species, the effects of indoxacarb and spinosad were bioassayed in the laboratory. The bioassays included exposure of adults to various aged residues on glass and cabbage leaf surfaces at different intervals after application, and direct spray on host eggs for effects on parasitism and development and mortality of parasitoid eggs, young and old larvae and pupae. The results showed that the glass- and leaf-surface residues of indoxacarb were harmless to both parasitoid species, whereas those of spinosad were moderately harmful to harmful to both parasitoid species depending on the rates used. The use of indoxacarb on host eggs did not affect significantly parasitism by both parasitoid species, whereas the higher rates of spinosad reduced parasitism. However, both insecticides did not affect immature development and adult emergence. Results from direct spray of host eggs with various immature stages inside showed that indoxacarb did not have significant effects on the egg, young and old larval stages and the pupal stage; whereas the high rates of spinosad when applied at the older larval and pupal stages significantly reduced adult emergence for both parasitoid species. Therefore, application of spinosad in an agro-ecosystem where Trichogramma are dominant should be carefully evaluated or avoided.