Induction of supernumerary larval moulting in the tobacco hornworm Manduca sexta: interaction of bisacylhydrazine ecdysteroid agonists with endogenous Juvenile Hormone

When given in a critical dietary dose range, the insecticidal bisacylhydrazine ecdysteroid agonists RH-5849 or tebufenozide (RH-5992) cause fifth stage Manduca sexta (L.) larvae to moult to a supernumerary sixth-stage giant larva. The effect is dependent on exposure to the chemicals immediately after the previous ecdysis. Previous removal of the corpora allata does not interfere with the induction of premature moulting by RH-5849 but completely prevents the formation of supernumerary larvae. The juvenilizing effect is therefore due to the interaction of the moult-promoting effect of the ecdysteroid agonists with the high titre of endogenous Juvenile Hormone that is present just after ecdysis to the fifth stage in this insect. The ecdysteroid agonists themselves appear to have no intrinsic Juvenile Hormone-agonist properties. Sixth-stage larvae resulting from exposure to critical dietary concentrations of RH-5849 are morphologically completely larval in character. When transferred to diet without the ecdysteroid agonist, they feed normally and gain weight, growing much larger than control fifth stage insects. At the end of the supernumerary stage, they cease to feed, wander in the usual way, and form a normal pupal cuticle but then die as pharate pupae without shedding the sixth-stage larval cuticle.