EFFECTS OF PARASITISM BY THE BRACONID WASP COTESIA-CONGREGATA ON METABOLIC-RATE IN HOST LARVAE OF THE TOBACCO HORNWORM, MANDUCA-SEXTA

We examined growth rates, gas exchange patterns and energy metabolism of tobacco hornworm (Manduca sexta) larvae parasitized by the braconid wasp Cotesia congragata. Larvae parasitized at the beginning of the f ourth-instar had reduced growth compared to unparasitized larvae of th e same age and short-term differences in metabolism (measured as rates of CO2 production, (V CO2) were apparent almost immediately after was p oviposition, However, over the growth period between parasitization and the last part of the fifth-instar, there was no significant differ ence between parasitized and unparasitized hosts as seen in the relati onship between mass and (V CO2, One day prior to parasitoid emergence, host larvae stopped eating, ceased spontaneous locomotor activity and showed a dramatic decline in metabolism, The 60% decline of(V CO2 at this time is consistent with lack of specific dynamic action because t he animals were not feeding, Gas exchange became highly cyclical on th e day of parasitoid emergence, but the cause and significance of this phenomenon, which disappeared by the third day following emergence, ar e not clear, This pattern of cycling was not induced by starving nonpa rasitized larvae for 6 days, nor by immobilizing nonparasitized larvae with tetrodotoxin. Ecdysteroid levels in the host's hemolymph signifi cantly increased on the day when parasitoids completed their L2-L3 mol t and began emerging, but not during the wasps' L1-L2 molt which occur red a few days earlier, Contrary to our initial expectation that hemol ymph ecdysteroid titers might be linked to alterations in the host's m etabolic rate, we observed no such correlation. (C) 1997 Elsevier Scie nce Ltd. All rights reserved.