Endocrine Activity retained in Diapause Insect Larvae

Abstract

DIAPAUSE is a state of arrested development which synchronises insect activity cycles with seasons when food resources are available. Facultative diapause occurs in response to environmental cues such as photoperiod and temperature which influence the endocrine system before the unfavourable conditions arrive^1,2. We have now obtained evidence that the currently accepted theory of humoral regulation of larval diapause should be revised. Past research on the pupal diapause of Saturniid moths led to the widely accepted conclusion that larval and pupal diapause result from a failure of the neurosecretory cells of the brain or the corpora cardiaca to release the brain (prothoracotropic) hormone which is required to stimulate the prothoracic glands to secrete the moulting hormone, ecdysone^3–5. The arrest of growth and moulting and the initiation of diapause have therefore been attributed to the absence of this brain hormone from the haemolymph⁶. Available evidence linking larval diapause with an inactive endocrine system has, however, remained inconclusive^7–10. Our data indicate that, on the contrary, mature diapause larvae of the southwestern corn borer, Diatraea grandiosella Dyar, must retain a functional endocrine system since they undergo stationary ecdyses during diapause. The ability of diapause larvae to undergo stationary ecdyses remains consistent with the concept of diapause as a state of arrested development because the stationary ecdyses do not affect the developmental stage¹¹. (Lüscher used the term stationary ecdysis to describe the ecdysis of the pseudergate larvae of the termite, Kalotermes flavicollis, when no progressive development occurs.) We now have evidence to implicate juvenile hormone (JH) in the regulation of this larval diapause.