Regulation of prothoracicotropic hormone release by juvenile hormone in the penultimate and last instar larvae of Mamestra brassicae.

The penultimate instar larvae of Mamestra brassicae have one ecdysteroid peak 2 days after ecdysis which induces larval ecdysis. Allatectomy deleted this peak and caused a larger peak 7 days later just before precocious pupation, in a pattern similar to the ecdysteroid titer of the last instar. Application of juvenile hormone (JH) immediately after allatectomy restored the normal larval pattern and larval ecdysis. The brain of the normal Day 1 or 2 penultimate instar larva had higher prothoracicotropic hormone (PTTH) activity than that of the allatectomized larva. Therefore, JH in the penultimate instar activates the brain to secrete and/or produce PTTH. Allatectomy of last instar larvae prolonged the period from gut purge to pupation by causing a delay of the ecdysteroid peak necessary for pupation. Application of JH restored the normal timing of ecdysteroid release. Thus, JH is important for the occurrence of the ecdysteroid surge on Day 7, since its absence delayed this surge by a day.