PRECOCIOUS INDUCTION OF METAMORPHOSIS IN SPODOPTERA-LITTORALIS (NOCTUIDAE) BY THE PARASITIC WASP CHELONUS-INANITUS (BRACONIDAE) - IDENTIFICATION OF THE PARASITOID LARVA AS THE KEY REGULATORY ELEMENT AND THE HOST CORPORA ALLATA AS THE MAIN TARGETS

The solitary egg-larval parasitoids of the genus Chelonus induce in th eir lepidopterous hosts precocious onset of metamorphosis followed by developmental arrest in the precocious prepupa. The parasitoid larva e merges as a freshly moulted 3rd instar larva from the precocious prepu pa. In the Chelonus inanitus-Spodoptera littoralis system, we recently showed by using both direct and indirect approaches that polydnavirus and venom induce developmental arrest in the prepupa but do not induc e precocious onset of metamorphosis [Seller and Lanzrein, J. Insect Ph ysiol., in press]. Here we show results of parasitoid removal experime nts and of experiments where parasitoid larvae were implanted into pol ydnavirus and venom containing larvae (in the absence of these compone nts implanted parasitoids were encapsulated within a few hours). All o ur data indicate that the presence of a late first instar parasitoid l arva is a prerequisite for precocious induction of metamorphosis in th e 5th stadium. Under experimental conditions, the parasitoid larva oft en also induced the formation of miniature 6th instar larvae, which ha ve a smaller head capsule width and reach a lower weight than nonparas itized larvae, suggesting that modification of threshold size for meta morphosis by the parasitoid larva is a gradual effect. Measurements of juvenile hormone biosynthesis by the corpora allata in vitro revealed that in parasitized larvae the corpora allata become precociously ina ctivated at the moult to the 5th stadium. Implantation of late first i nstar parasitoid larvae into 4th instar polydnavirus and venom contain ing larvae led to inactivation of the recipient's corpora allata. Thes e results suggest that in this parasitoid-host system factor(s) from t he late first instar parasitoid larva inactivate the host's corpora al lata which in turn leads to a precocious onset of metamorphosis. (C) 1 996 Wiley-Liss, Inc.