Test cohorts of the parasitoid Cotesia plutellae ( Hymenoptera: Braconidae) were reared on a strain of the host, Plutella xylostella, with a high level of resistance to the insecticide spinosad. To observe the lethal and sublethal effects of the insecticide on various stages of C. plutellae, cohorts of the parasitoid were treated with 25 mg/L spinosad ( the recommended rate for field application) at the egg or larval stage by applying the insecticide to the host larvae, or at the pupal stage by direct application. In addition, the toxicity of the insecticide on adult wasps was bioassayed. Compared to the controls, when the parasitoids were exposed to spinosad at the egg or larval stage via their host larvae, they suffered mortality within the hosts or in the process of egression out of the hosts, resulting in reduction in cocoon formation and pupation by 43%[approximately]56% , adult emergence by 19%[approximately]33% and adult female ratio by 11%[approximately]25%, while adult body size, longevity and ability of parasitism were unaffected; when the parasitoids were exposed to spinosad at the pupal stage, adult emergence was reduced by 10% and adult longevity was decreased, while ability of parasitism of the adult females was unaffected; and the LC99 for adults at 24 h tested by residue contact was 4.31 mg/L. These results demonstrate that spinosad can exert toxicity on the parasitoid eggs and larvae following entry to the host larvae, or exert toxicity directly on the parasitoid pupae and adults , thereby cause lethal and sublethal effects on all stages of the parasitoid, with the highest acute toxicity to the adult stage. Plutella xylostella (Plutellidae) : Host Cotesia plutellae (Braconidae) : Parasite Biometrics; Life cycle and development; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Population dynamics; Population structure; Abiotic factors; Chemical factors Cotesia plutellae; Size; Body size; Life cycle; Life history; Lepidopteran hosts; Insecticide effect on parasite body size; life history & population sex ratio; Plutella xylostella; Population sex ratio; Fertilizers and pesticides; Spinosad insecticide Plutella xylostella; Hymenopteran parasites; Insecticide effect on parasite body size; life history & population sex ratio; Cotesia plutellae; Fertilizers and pesticides; Spinosad insecticide none