Effect of release of introduced ichneumonid parasitoid, Diadegma semiclausum (Hellen) on Diamondback Moth, Plutella xylostella (L) in experimental cabbage field

Diadegma semiclausum was introduced from Taiwan to Japan for the first time in 1989 and was released into an experimental farm from 1991 to 1993. Each release consisted of 100 females. The single release experiment in 1991 produced 30% parasitism of the Diamondback Moth in 1 month. In 1992, four releases were made at intervals of 10 days, and the parasitism reached 50% after 30 days and 70% after 50 days. In 1993, a similar experiment was repeated by increasing the number of releases from four to eight, and starting 1 month earlier. The resultant parasitism was 70%. In 1991, the sex ratio of the D. semiclausum was close to 1.0 during the first 10 days, falling to 0.25. In the 1993 experiment, the populations of the four indigenous natural enemies of the Diamondback Moth decreased as the parasitism by D. semiclausum increased.