Abstract
The effect of four insecticides, which is resistered for use on cabbage in Japan, on Diadegma semiclausum, a larval parasitoid of the diamondback moth, Plutella xylostella, was assessed by four different bioassays, i.e., (1) dry film assay, (2) semifield assay in the cage, (3) direct spray assay for parasitoid cocoons, and (4) parasitizing assay using females that survived the dry film assay. The results showed that chlorfluazuron (IGR) and the microbial toxin from Bacillus thuringiensis var. kurstaki had minimun effects on D. semiclausum in all the assays. Ethofenprox (pyrethroid) showed the highest insecticidal activity in the dry film assay (100%, female mortality), however moderate mortality was observed in the semifield assay (32%) and in the direct spray assay (76% of cocoons could have emerged). Pyrazole chlorfenapyr showed moderate toxicity to D. semiclausum in all the assays. These results suggest that an inappropriate assay could lead to an erroneous conclusion on the actual effect of an insecticide on natural enemies, such as D. semiclausum, which exhibits high locomotive ability and some repellency to insecticides under field conditions.
