Juvenile hormone titres and metabolism during starvation-induced supernumerary larval moulting of the tobacco hornworm, Manduca sexta L.

When tobacco hornworm (manduca sexta) larvae are starved for 5 days immediately after ecdysis to the 5th instar, then fed normal diet, they undergo a supernumerary moult instead of metamorphosis. During starvation the titre of juvenile hormone in the haemolymph increased to a maximum of 3 ng juvenile hormone I equivalents/ml (determined by the black Manduca larval bioassay) on the fourth day of starvation, then began a decline which continued through the subsequent feeding period. The changes in juvenile hormone titre were not attributable to changes in haemolymph volume during starvation (only a 5% decrease) and subsequent feeding. During starvation the esterase activity of the haemolymph declined 4-fold with a 2-fold larger decrease in the DFP-insensitive, presumably juvenile hormone specific, esterase activity. Both the total and the juvenile hormone-specific esterase activity then increased as a function of larval weight during the subsequent feeding period. As growth was slow in the prolongedly starved larvae, sufficient juvenile hormone was present at the time of prothoracicotropic hormone (PTTH) and ecdysteroid release at the beginning of the fourth day of feeding to prevent metamorphosis.