Removal of the prothoracic glands early during the 5th instar of Manduca sexta prevented the larvae from wandering and from further development. Infusion of 20-hydroxyecdysone (20-HE) into these larvae induced wandering behaviour. In intact larvae, induction of precocious wandering behaviour required a 20-HE infusion lasting longer than 5 h. Infused 20-HE induced maximal response (90%) when delivered at a rate of 0.06 micrograms g-1 body weight h-1. At considerably higher concentrations (0.25 micrograms g-1 h-1 larvae performed brief, erratic behaviour or omitted wandering entirely. The latency between appearance of 20-HE and the onset of wandering was dose-dependent with a minimum of 11 h following infusion at 0.1 micrograms g-1 h-1. Latency was not affected by the duration of 20-HE infusion. The duration of induced wandering behaviour was proportional to the duration of 20-HE infusion. Minimal wandering behaviour lasted 2 h following 20-HE infusions at 5 h, while infusions lasting 11 h induced 9 h of wandering behaviour. Several lines of evidence suggest that the effects of 20-HE accumulate over time and directly determine the duration of wandering behaviour. Many larvae exhibited a series of temporally distinct locomotor periods following various 20-HE infusion protocols, suggesting that a series of separate exposures to 20-HE can result in corresponding serial bouts of locomotion. Responsiveness to 20-HE appeared to be principally modulated by juvenile hormone. Allatectomy of 2nd, 3rd and 4th instar larvae removed juvenile hormone (JH) precociously from these stages and was followed several days later by precocious wandering behaviour. Likewise, application of the JH mimic, EGS, prior to 20-HE exposure or at the start of it, could prevent the behavioural induction. During the 5th instar, 20-HE became increasingly effective in inducing wandering as larvae grew larger than 5 g, the size at which JH normally begins to disappear from the haemolymph. Allatectomized 5th instar larvae responded directly to 20-HE a day sooner than larvae with normal JH titres, but before day 2 the effects of 20-HE on wandering behaviour appear to be indirect, requiring a latency greater than 24 h. Several processes, of which the elimination of JH is the last, appear to be required before 20-HE can induce wandering behaviour.(ABSTRACT TRUNCATED AT 400 WORDS)
The release of the prothoracicotropic hormone in the tobacco hornworm, Manduca sexta, is controlled intrinsically by juvenile hormone