After-release dispersal of Diadegma semiclausum and its effect on diamondback moth population and damage, and indigenous parasitoids

摘要

Diadegma semiclausum, an exotic parasitoid of the diamondback moth, was introduced and released in the highlands of Kenya, northern Tanzania and southwest Uganda. The natural spread of the parasitoid from one of the release sites in Kenya, 27 months after the release, was assessed through collections made at equidistant points in four cardinal directions. The parasitoid was found up to a distance of 30 km from the release site. Wherever D. semiclausum was collected, it was the major parasitoid species. Diamondback moth populations were slightly lower near the release site than at distant collection points. Indigenous parasitoids collected were Oomyzus sokolowskii ( Hym. : Eulophidae) , Diadegma mollipla ( Hym. : Ichneumonidae) and Apanteles sp. Foerster ( Hym. : Braconidae) . Overall parasitism by D. semiclausum, the introduced parasitoid, was more prevalent than by indigenous parasitoids. In Tanzania and Uganda, the results of a field survey conducted 24 and 21 months after release of D. semiclausum, respectively, indicate that the overall parasitism and the contribution of the introduced parasitoid to the control of the diamondback moth population decreased with increasing distance from the release point. In Tanzania, the diamondback moth population declined from 6. 1 per plant, recorded eight months after introduction of D. semiclausum, to 3. 2 per plant. At the release site in Tanzania, the percentage parasitism was over 70 percent but this declined to 47 percent at more than 10 kilometres away from the release site. Within the release area in Uganda, diamondback moth population declined from 1. 8 per plant recorded before release, to 0. 7 twenty-one months after introduction of D. semiclausum. Percentage parasitism was over 50 percent but this declined to 31 percent at more than 20 kilometres away from the release site. In both countries, this was attributed to the lack of farmers' understanding of the biological control activities and the continuation of insecticide use. A farmer education programme is recommended to improve the situation.