Endocrine Mechanisms Regulating Post-Diapause Development in the Cabbage Armyworm, Mamestra brassicae

Diapause, a programmed developmental arrest at a specific stage, is common in insects and is regulated by hormones. It is well established that in pupal diapause, cessation of ecdysteroid secretion from the prothoracic glands (PGs) after pupal ecdysis leads to diapause initiation, while resumption of its secretion induces post-diapause development. However, what regulates the activity of the glands is poorly understood, especially for the glands of diapause-terminated pupae. In the present study, we investigate the mechanisms by which post-diapause development is regulated in the cabbage armyworm Mamestra brassicae. We demonstrate that the brain is necessary for the initiation of post-diapause development and that the factor in the brain responsible for the activation of the PGs is the prothoracicotropic hormone (PTTH). Further, through measuring the hemolymph PTTH titers by time-resolved fluoroimmunoassay, we show that PTTH is actually released into the hemolymph prior to the activation of the PGs. Although its peak titer is much lower than expected, this low concentration of PTTH is most likely still effective to activate the PGs of post-diapause pupae, because the responsiveness to PTTH of the glands at this stage is very high compared to that of nondiapause pupal PGs. These results strongly suggest that in M. brassicae, PTTH serves as a trigger to initiate pupa-adult development after diapause termination by stimulating the PGs to secrete ecdysteroid.