Publisher Summary
This chapter briefly discusses hormonal control of diapause. Diapause is an arrest in development accompanied by a major shutdown in metabolic activity. The evolution of diapause is arguably one of the most critical events bolstering the success of insects. The capacity to periodically shut down development has enabled insects and their arthropod relatives to invade environments that are seasonally hostile. Indeed, very few environments permit continuous insect development. Even in the tropics, where temperatures throughout the year may be compatible with ectotherm development, seasonal patterns of rainfall drive cycles of plant growth that favor insects with the ability to periodically become dormant. Unlike a simple quiescence that is an immediate response to an unfavorable environmental condition, diapause is a genetically programmed response that occurs at a specific stage for each species. Sometimes, the insect will enter diapause at this particular stage in each generation regardless of the environmental conditions it receives—a developmental program referred to as obligatory diapause. Much more frequently, the decision to enter diapause is determined by environmental factors, usually day length, received by that individual or its mother at an earlier developmental stage. This is referred to as facultative diapause.

10 – Hormonal Control of Diapause

This review examines recent findings about the endocrine basis of larval diapause. Evidence has accumulated to support the view that larval diapause is regulated.by distinctive endocrine interactions. While the lack of the molting hormone, ecdysone, causes the arrest of morphogenesis in both diapausing larvae and pupae, the mecha­ nism leading to the inactivity of the ecdysial glands appears to differ between these life stages. The recent discovery of a juvenile hormone (JH) involvement in the regulation of the larval diapause of two species (24, 144) indicates that at least some species of diapausing larvae retain activity within their endocrine system. This finding has underscored the need to further examine the extent of endocrine activity during larval diapause. For background information, several reviews are available on insect endocrinology (36, 44, 139) and on the physiology and ecology of diapause (35, 87, 121). Hormonal failure and JH regulation are the two principal theories which have been developed to explain larval diapause. The hormonal failure theory was devel­ oped originally to explain the endocrine basis of pupal diapause (136138), and proposes that diapause is instituted when the cerebral neurosecretory (NS) system becomes inactive and the corpora cardiaca stop releasing the ecdysiotropin that is required to stimulate the ecdysial glands to secrete ecdysone. Morphogenesis resumes after the corpora cardiaca renew their release of ecdysiotropin. The dia­ pause state, therefore, is thought to be distinguished by complete inactivity in the endocrine system, including the corpora allata. This theory has been extended to explain the endocrine basis of larval diapause (56, 7 1, 135). Conversely, the JH regulation theory proposes that diapausing larvae, unlike diapausing pupae, retain actively secreting corpora allata and that JH initiates and maintains diapause by regulating the secretion of ecdysiotropin (24, 144). The corpora allata are thought to be controlled by ordinary and NS neurones from the brain. An intermediate titer of JH associated with diapause is believed to arrest morphogenesis by inhibiting

Hormonal regulation of larval diapause

Juvenile hormone regulation of the larval diapause of the Southwestern corn borer, Diatraea grandiosella☆

Photoperiodic and thermoperiodic interactions in the regulation of the larval diapause of Diatraea grandiosella

Changes in haemolymph juvenile hormone (JH) concentrations of larvae of the southwestern corn borer, Diatraea grandiosella, were used to estimate the activity of the corpora allata. The haemolymph of penultimate nondiapause instars contained a maximum JH titre of 3000 Galleria units (GU)/ml, whereas the titre in the final instar had dropped to 140 GU/ml within 6 h of the ecdysis and continued to remain low. Corpora allata of last-stage non-diapause larvae therefore remain inactive. Some signal other than a rapidly declining JH titre in mid-instar triggers the release of ecdysiotropin because the pupal moulting cyclic did not become independent of the brain and ecdvsial glands until 4–25 and 4–75 days after the final larval ecdysis, respectively. Newly diapaused immaculate larvae had a haemolymph JH titre of ca. 1500 GU/ml. Three JH mimics applied to early last stage non-dispause larvae induced the formation of the immaculate dispause morph. The mimics did not activate the recipient's corpora allata, but elevated the function JH concentration, each having a different intrinsic hormonal activity. Threshold JH concentrations which control larval ecdyses, dispause maintenance, and metamorphosis are presented.

Horomonal control of larval diapause and metamorphosis of the southwestern corn borer Diatraea grandiosella

Immaculate larvae which lack cuticular pigmentation are the typical diapause larvae of Diatraea grandiosella Dyar. Results snowed that mature spotted larvae reared under diapause-inducing conditions underwent a stationary ecdysis to become immaculate larvae. This stationary ecdysis occurred earlier when mature larvae were held at 30°C than at 23°C. Mature spotted larvae had a high weight loss until the immaculate ecdysis, after which their weight decreased slowly. During this period mature larvae became partially dehydrated and 10 days after the immaculate ecdysis had a mean water content of only 65%. A comparison of laboratory-reared and field-collected spotted and immaculate larvae revealed no physiological differences. However, field larvae were heavier and contained a significantly higher concentration of hemolymph pigments than the laboratory ones. These pigments were found to be carotenoids which were absorbed from ingested host plant tissues. The possible nature of hormonal mechanisms regulating diapause induction and development are discussed.

A. S. Reddy

Papers

Diapause of the Southwestern Corn Borer, Diatraea grandiosella: Transition from Spotted to Immaculate Mature Larvae