Toxicity and kinetics of spinosad in different developmental stages of the endoparasitoid Hyposoter didymator (Hymenoptera: Ichneumonidae) and its host Spodoptera littoralis larvae (Lepidoptera: Noctuidae)

Abstract

The toxicity of spinosad was evaluated using the RaPID Assay® Spinosad immunosorbent assay in different developmental stages of the parasitoid, Hyposoter didymator, and in its host, fourth-instar larvae of the cotton leafworm Spodoptera littoralis. Spinosad was applied directly to pupae and adults of H. didymator (ingestion or topical application) or to the immature stages of the parasitoid via the host larvae. Low amounts of spinosad were recovered from S. littoralis host larvae after topical treatment, and the compound was mainly retained in the hemolymph. Amounts of spinosad detected in third-instar larvae of H. didymator, pulled out from the hemolymph of parasitized S. littoralis larvae, were 85 pg (3.57 ng a.i./g body weight) in dead larvae, and 82 pg (3.42 ng a.i./g body weight) in alive individuals. After topical treatment of H. didymator cocoons, most of the compound was retained in the silken cocoon, preventing contamination of the pupa. Also in the parasitoid adults, relatively low amounts of spinosad were accumulated in the body overall, but half of all the insecticide recovered was found in the ovaries. The kinetic results obtained help to better understand the toxicity of spinosad in the complex S. littoralis–H. didymator, and to ascertain the compatibility between spinosad and the parasitoid for optimizing the control of lepidopteran pests.