6.5. Diapause

The developmental progression from egg to adult often is interrupted by a period of dormancy. This occurs particularly in temperate areas when environmental conditions become unsuitable, such as in seasonal extremes of high or low temperatures, or drought. Dormancy may occur in summer (aestivation (estivation)) or in winter (hibernation), and may involve either quiescence or diapause. Quiescence is a halted or slowed development as a direct response to unfavorable conditions, with development resuming immediately favorable conditions return. In contrast, diapause involves arrested development combined with adaptive physiological changes, with development recommencing not necessarily on return of suitable conditions, but only following particular physiological stimuli. Distinguishing between quiescence and diapause requires detailed study.

Diapause at a fixed time regardless of varied environmental conditions is termed obligatory. Univoltine insects (those with one generation per year) often have obligatory diapause to extend an essentially short life cycle to one full year. Diapause that is optional is termed facultative, and this occurs widely in insects, including many bi- or multivoltine insects in which diapause occurs only in the generation that must survive the unfavorable conditions. Facultative diapause can be food induced: thus when summer aphid prey populations are low the ladybird beetles Hippodamia convergens and Semidalia unidecimnotata aestivate, but if aphids remain in high densities, as in irrigated crops, the predators will continue to develop without diapause.

Diapause can last from days to months or in rare cases years, and can occur in any life-history stage from egg to adult. The diapausing stage predominantly is fixed within any species and can vary between close relatives. Egg and/or pupal diapause is common, probably because these stages are relatively closed systems, with only gases being exchanged during embryogenesis and metamorphosis, respectively, allowing better survival during environmental stress. In the adult stage, reproductive diapause describes the cessation or suspension of reproduction in mature insects. In this state metabolism may be redirected to migratory flight (section 6.7), production of cryoprotectants (section 6.6.1), or simply reduced during conditions inclement for the survival of adult (and/or immature) stages. Reproduction commences post-migration or when conditions for successful oviposition and immature stage development return.

Much research on diapause has been carried out in Japan in relation to silk production from cultured silkworms (Bombyx mori). Optimal silk production comes from the generation with egg diapause, but this conflicts with a commercial need for continuous production, which comes from individuals reared from non-diapausing eggs. The complex mechanisms that promote and break diapause in this species are now well understood. However, these mechanisms may not apply generally, and as the example of Aedes below indicates, several different mechanisms may be at play in different, even closely related, insects, and much is still to be discovered.

Major environmental cues that induce and/or terminate diapause are photoperiod, temperature, food quality, moisture, pH, and chemicals including oxygen, urea, and plant secondary compounds. Identification of the contribution of each may be difficult, as for example in species of the mosquito genus Aedes that lay diapausing eggs into seasonally dry pools or containers. Flooding of the oviposition site at any time may terminate embryonic diapause in some Aedes species. In other species, many successive inundations may be required to break diapause, with the cues apparently including chemical changes such as lowering of pH by microbial decomposition of pond detritus. Furthermore, one environmental cue may enhance or override a previous one. For example, if an appropriate diapause-terminating cue of inundation occurs while the photoperiod and/or temperature is “wrong”, then diapause may not break, or only a small proportion of eggs may hatch.

Photoperiod is significant in diapause because alteration in day length predicts much about future seasonal environmental conditions, with photoperiod increasing as summer heat approaches and diminishing towards winter cold (section 6.10.2). Insects can detect day-length or night-length changes (photoperiodic stimuli), sometimes with extreme accuracy, through brain photoreceptors rather than compound eyes or ocelli. The insect brain also stores the “programming” for diapause, such that transplant of a diapausing moth pupal brain into a non-diapausing pupa will induce diapause in the recipient. The reciprocal operation causes resumption of development in a diapausing recipient. This programming may long precede the diapause and even span a generation, such that maternal conditions can govern the diapause in the developing stages of her offspring.

Many studies have shown endocrine control of diapause, but substantial variation in mechanisms for the regulation of diapause reflects the multiple independent evolution of this phenomenon. Generally in diapausing larvae, the production of ecdysteroid molting hormone from the prothoracic gland ceases, and JH plays a role in termination of diapause. Resumption of ecdysteroid secretion from the prothoracic glands appears essential for the termination of pupal diapause. JH is important in diapause regulation in adult insects but, as with the immature stages, may not be the only regulator. In larvae, pupae, and adults of Bombyx mori, complex antagonistic interactions occur between a diapause hormone, originating from paired neuro-secretory cells in the suboesophageal ganglion, and JH from the corpora allata. The adult female produces diapause eggs when the ovariole is under the influence of diapause hormone, whereas in the absence of this hormone and in the presence of juvenile hormone, non-diapause eggs are produced.

Chapter 6