Submitted to: Experimental Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/2/1996
Publication Date: N/A
Citation: Interpretive Summary: The use of wasp parasitoids to control agricultural insect pests is tantalizing because it is ecologically sound and would reduce the use of chemical pesticides. Artificial rearing of parasitoids for release in the field is hampered by a lack of knowledge concerning host/parasite interactions. Previous studies have shown that Cotesia congregata, a braconid wasp, is able to control host (Manduca sexta) endocrine mechanisms so that conditions are favorable for its own development and emergence from the host. Developmental arrest and eventually death is induced in last stage host larvae. In these studies, the parasitoid was found to have a profound effect on host metabolism just prior to the emergence of the parasitoid, and to initiate a cyclical release of carbon dioxide on the day of wasp emergence. This is the first report of the occurrence and timing of parasitoid-induced changes on host metabolic rates. In addition, an ecdysteroid peak in host hemolymph was found to accompany the parasitoid's final molt, but not its molt from a first to a 2nd stage larva. The data should facilitate the appropriate manipulation of environmental conditions for the artificial rearing of parasitoids of economic importance.
Technical Abstract: Tobacco hornworm larvae that are parasitized by the braconid wasp Cotesia congregata at the beginning of the fourth instar weighed less in the fifth instar compared to nonparasitized larvae of the same chronological age. The day before the wasps were due to emerge, there was a linear positive correlation between the final mass of the host-parasite complex and the number of parasitoids present in the host. Differences in the rates of carbon dioxide emission between parasitized and nonparasitized larvae were seen beginning almost immediately after the host was parasitized, with the most exaggerated differences seen after the host completed molting to the fifth instar. The metabolic rate of the host dropped dramatically during the final days of parasitism just before the wasps emerged from the host larva, when the host's rates of food consumption and spontaneous locomotion decreased markedly. The drop in metabolic rate seen after emergence likely reflected the dramatic decrease of energy needed for food processing and assimilation or specific dynamic action (SDA) following cessation of feeding. Importantly, a consistent cyclical pattern of carbon dioxide emission was noted on the day when the wasps were emerging from the host, with bursts of carbon dioxide emission occurring every 4-10 minutes. A significant increase in the host's hemolymph ecdysteroid titer was detected on the day when the parasitoids completed their L2-L3 ecdysis and began emerging, but not during the wasps' ecdysis from the first to the second instar, which occurred a few days earlier.