Ecdysteroids and Insect Host/Parasitoid Interactions

Hemolymph concentrations of immunoreactive ecdysteroids were estimated by radioimmunoassay in healthy and parasitized larvae. There was a significantly (P < 0.001) higher concentration of ecdysteroids in the hemolymph of hosts with second stadium parasitoids than in hosts with first stadium parasitoids. Maximum host hemolymph titers of ecdysteroids occurred just prior to parasitoid eclosion, similar to the hormonal milieu found in healthy prepupae. Parasitized larvae exhibited prepupal behavior and morphology at the time of parasitoid egression from the host. Parasitoid larvae excised from hosts and held in vitro released immunoreactive ecdysteroids into Grace's medium. In vitro release was constant over a 24-h incubation period. Our hypothesis that natural selection pressures have favored egg/larval parasitoids capable of castrating their hosts is discussed. Embryonic castration eliminates competition of host testes for limited host nutrients and/or hormones. Competition may be especially acute following a prolonged period of dormancy. (C) 1991 Academic Press, Inc.