Seasonal occurrence of diamondback moth, Plutella xylostella (L.) and parasitoids in some commercial cruciferous fields in Miyazaki, southern Japan

A total of seven parasitoid species emerged from larvae and pupae of the diamondback moth collected in commercial crucierous fields in Miyazaki. Among the parasitoids, Cotesia plutellae (a solitary larval parasitoid), Oomyzus sokolowskii (a gregarious larval-pupal parasitoid), and Diadromus subtilicornis (a solitary pupal parasitoid) were dominant. D. subtilicornis and O. sokolowskii were most common in spring-summer and summer-autumn, respectively. C. plutellae appeared in all seasons except winter. The percentage parasitism by C. plutellae, O. sokolowskii, and D. subtilicornis was about 70%, 50% to 60%, and >40%, respectively, when no or almost no insecticide was sprayed on the cruciferous fields. One to four applications per month of insecticides such as dichlorvos, fenitrothion, fenvalerate, BPMC, and methomyl strongly lowered the parasitoid densities in the fields, but the biotic insecticide, BT, had no effect.