Synthesis/release of ecdysteroids by Cotesia congregata, a parasitoid wasp of the tobacco hornworm, Manduca sexta

Parasitized Manduca sexta undergo developmental arrest in the instar (typically the fifth) from which the parasitoid wasp Cotesia congregata will emerge. Host hemolymph ecdysteroid titers as well as levels of other regulatory molecules are manipulated by the parasitoid to create an environmental milieu which is most suitable for the wasp to complete its life cycle. Based on the results of in vivo studies (reported earlier), the parasitoid appears to require a host hemolymph ecdysteroid titer of 300-400 pg/microliter, 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. Ecdysteroid content of whole body homogenates increased with parasitoid age. Body length, anal vesicle size and time of ecdysis provided more precise indicators of times when ecdysteroid content would be high or low. 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 also increased with parasitoid age. When body length was the independent variable, 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, 20HE and polar ecdysteroids were the predominant ecdysteroids released on days 1 through 4. Thus, an ecdysteroid known to be physiologically active is released by the parasitoid. 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 including parasitoid size and instar, size/degree of ballooning of the anal vesicle, and shedding of the 1st instar cuticle.