Density suppression of Plutella xylostella (Lepidoptera:Yponomeutidae) by multiple releases of Diadegma semiclausum (Hymenoptera

The exotic parasitoid, Diadegma semiclausum, was introduced and augmented to suppress the density of the diamondback moth, Plutella xylostella in cabbage fields at two localities in Iwate Prefecture, northern Japan, from 1996 to 1999. The parasitoids were released at densities of 0.8-3.4 copulated females per plant. In 1996, the numbers of both fourth-instars and pupae of P.xylostella in the field where parasitoids were released remained below five individuals per plant from release until harvest in late July. This was much less than the levels in the control field, where the peak density was 17.8 larvae and 11.2 pupae per plant. Percentage parasitism by D. semiclausum reached 53% in the release field in mid June. Successful overwintering of D. semiclausum in the northern Tohoku district of Japan was observed in the survey conducted from June to July of 1997. However, the percentage parasitism was reduced to a maximum of 29% in 1997 when no additional parasitoids were released. The release of the parasitoid was repeated in Nishine in 1997 and in Morioka in 1998 and 1999, resulting in successful suppression of P. xylostella.