AbstractThe braconid endoparasitoid,Cotesia (=Apanteles) kariyaiphysiologically influences its host,Pseudaletia separata, through three factors: polydnavirus, venom, and teratocytes. Inhibiting testis development of the host seems to be one factor that is important for successful development of the parasitoid. CkPV (polydnavirus ofCotesia kariyai)plus venom depressed testis development. Testes from unparasitized day 0 last instar transplanted into isolated abdomens increased in volume after stimulation with 20‐hydroxyecdysone (20HE). However, day 0 testis preincubated with CkPV plus venom for 6 h and then transplanted into an isolated abdomen did not respond to 20HE. Southern blot analysis indicated CkPV‐DNA hybridized to testes‐DNA from parasitized hosts, suggesting the possibility that CkPV is involved in suppression of testes growth. Binding assays using PNA indicated a 2‐fold increase in ecdysteroid receptor binding activity during the late stage of parasitism. The increase in receptor activity might be related to the maintenance of a low ecdysteroid titer in parasitized hosts due to a feedback response. © 1994 Wiley