Physiological effects of parasitism by Apanteles congregatus in terminalstage tobacco hornworm larvae

Abstract

The physiology of Manduca sexta (L.) larvae undergoes several modifications during the final stages of parasitism by the gregarious braconid wasp Apanteles congregatus (Say). Normal unparasitized larvae experience a drastic decline in the level of haemolymph juvenile hormone during the fifth instar that acts as a stimulus for metamorphosis to gegin. In contrast, the hormone titre of parasitized fifth-instar larvae remains high enough to inhibit metamorphosis of the host, probably due to increased rates of hormone biosynthesis combined with decreased degradation by juvenile hormone-specific esterase in the host. Although the haemolymph ecdysteroid titre rises to the normal prewandering level, it increases in the presence (as opposed to the normal absence) of juvenile hormone in parasitized fifth-instar larvae. In vitro assays of ecdysone 20-monooxygenase levels in homogenates of fat body and midgut show little activity exists during the latter phases of the fifth instar in conjunction with the major ecdysteroid peak coinciding with emergence of the wasps; as a consequence the prohormone ecdysone may not be efficiently converted to 20-hydroxyecdysone. After emergence, the lack of moulting of the host larva can be attributed to low titres of haemolymph ecdysteroid and ecdysteroid and ecdysone 20-monooxygenase. Parasitism also inhibits the growth, food consumption, and locomotor activity of terminal stage fifth-instar hosts. Debraining host larvae on day 4, however, causes the host to develop a hyperkinesis mimicking wandering, indicating the neural and muscular connections necessary for this behaviour are intact despite the normal inhibitory influence of the brain. Parasitism-specific proteins are detectable in the haemolymph of all terminal-stage hosts that are not seen in unparasitized larvae of the same age. The molecular weights of two of the most prominent proteins are 56 and > 120 kd, respectively, as determined by SDS gel electrophoresis. Haemolymph arylphorin titres also appear affected by parasitism.