Effects of a Selective and a Broad-Spectrum Insecticide on Parasitism Rates ofPlutella xylostella(L.) (Lepidoptera: Plutellidae) and Species Richness of Its Primary Parasitoids

Abstract

The direct impact of broad-spectrum insecticides on primary parasitoids is considered a major contributing factor to the high pest status of diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), in many parts of the world. As a result, selective insecticides are often put forward as a solution to the problem of integrating chemical and biological control methods. However, there is paucity of studies that have directly contrasted effects of selective and broad-spectrum insecticides on parasitism rates and parasitoid species richness in the field. We compared effects of weekly and bi-weekly application regimes of a selective insecticide (Dipel®) and a broad-spectrum insecticide (dichlorvos) on parasitism rates of P. xylostella and species richness of its primary parasitoids against unsprayed control for two cropping seasons. Parasitoids were reared from immature P. xylostella in all treatments, and parasitism rates were not significantly different among the treatments. During October–December 2011, four species of primary parasitic Hymenoptera [Cotesia vestalis (Haliday) (Braconidae), Apanteles halfordi (Ullyett) (Braconidae), Oomyzus sokolowskii (Kurdjumov) (Eulophidae), and Diadromus collaris (Gravenhorst) (Ichneumonidae)] were reared from P. xylostella larvae and pupae, whereas three parasitoid species (C. vestalis, O. sokolowskii and D. collaris) were reared during March–May 2012. Cotesia vestalis accounted for >80%of total parasitism rates in all treatments. In both seasons, parasitoid species richness was highest on the control treatment. Although two parasitoid species were recorded on all Dipel® and dichlorvos treatments during October–December 2011, only one parasitoid species was recorded on Dipel® treatments during March–May 2012 compared to two species on dichlorvos treatments. Thus, insecticide application regime had no influence on parasitoid species richness, instead insecticide type did. Since P. xylostella infestations were significantly lower in Dipel® treatments in both seasons, this study suggests that a greater impact of a selective insecticide on the pest population density can affect parasitoid species richness more than the direct impact of a broad-spectruminsecticide with a short crop residual period.