Submitted to: Archives of Insect Biochemistry and Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/1998
Publication Date: N/A
Citation: Interpretive Summary: The use of wasp parasitoids to control agricultural insect pests is ecologically sound and as part of an IPM program, should reduce the use of chemical pesticides. Cotesia congregata is a braconid wasp which induces developmental arrest in its host, the tobacco hornworm. The mechanism(s) of action used by the parasite to manipulate host molting hormone levels to create conditions that are favorable for parasite development are not known. In these investigations, we report that the wasp both synthesizes and releases molting hormone into its host's hemolymph, and that the increased hormone levels in the host appear to be necessary for the wasp's final larval molt and emergence from its host. In addition, we developed a useful system of markers for tracking wasp development and marking important physiological events in the wasp life cycle. Results of this research clarify the role of the parasitoid in manipulating host hormone levels. Information generated should be useful to other scientists investigating the mechanism(s) of action by which parasites induce developmental arrest in their hosts. The ultimate goal is to capitalize on the techniques used by parasitic wasps to inhibit insect growth and development in order to design new biorational strategies for controlling pest insects.
Technical Abstract: Parasitized Manduca sexta undergo developmental arrest in the instar (typically the fifth) from which the parasitoid wasp Costesia congregata will emerge. The parasitoid appears to require a host hemolymph ecdysteroid titer of 300-400 pg/ul which it effects in its host, in part, by synthesizing and releasing ecdysteroid. Here we used in vitro incubation to characterize C. congregata's synthesis and release of this group of steroid hormones. In addition, we identified physical characteristics as markers for tracking parasitoid ecdysteroid content as well as hormone synthesis and release. Body length, anal vesicle size and time of molt provided more precise indicators of times when ecdysteroid content would be high or low than did parasitoid age. Ecdysteroid levels peaked at the time of the parasitoid's molt from the 1st to the 2nd instar and again in the largest parasitoids that were preparing for their molt to the 3rd instar. Ecdysteroid synthesis and release peaked in larvae that were 3.0 mm (often undergoing a molt), dipped in larvae that were 3.6 mm (post-molt) and rose again as larvae increased in length. E, physiologically active 20HE and polar ecdysteroids were the predominant ecdysteroids released on days 1 through 4. This is the first time that a precise, detailed system of markers has been identified for tracking C. congregata development. Our results support the view that parasitoid synthesis and release of ecdysteroids are associated with specific events in the parasitoid's life cycle and can be predicted by selected physical markers.