5. Diapause in Aquatic Insects, with Emphasis on Mosquitoes

Diapause is the primary factor synchronizing insect life cycles with seasonal changes in the environment. Diapause is the major factor regulating the timing of growth, development, and reproduction, both before and after the period of dormancy. Many definitions of diapause may be found in the literature. Tauber et al. (1986) define diapause as “a neurohormonally mediated, dynamic state of low metabolic activity. Associated with this is a reduced morphogenesis, increased resistance to environmental extremes, and altered or reduced behavior activity. Diapause occurs during genetically determined stage(s) of metamorphosis, and its full expression develops in a species-specific manner, usually in response to a number of environmental stimuli that precede unfavourable conditions. Once diapause has begun, metabolic activity is suppressed even if conditions favorable for development prevail.” Aquatic insects are represented in many orders including the Diptera (Culicidae, Chironomidae, Simuliidae, Ceratopogonidae, Chaoboridae, Dixinae), Coleoptera, (Dytiscidae, Hydrophylidae), and Heteroptera (about 20 families); some orders such as Odonata, Ephemeroptera, and Trichoptera are exclusively aquatic. In most cases only immature stages of insects are true water inhabitants whereas the adults are terrestrial. Hibernation, diapausing stages, and induction and termination of diapause are studied in the mentioned insect groups to varying degrees. Among the aquatic insects, the most voluminous literature is devoted to the mosquitoes because of their medical and veterinary importance, thus the mosquitoes were selected as a model group for this discussion of diapause and controlling environmental factors. Diapause in other aquatic insects will be reviewed briefly at the end of this chapter. Though aquatic insects occur in specific habitats that cool more slowly in autumn, metamorphosis appears to be controlled by cues similar to those used by terrestrial insects. The photoperiod and temperature responses of aquatic insects are as broad as the range found in terrestrial species, though there may be greater flexibility in overwintering instars (Danks 1978). In mosquitoes the environmental control of diapause has been investigated from the early 1960s coinciding with the beginning of intensive studies in the field of seasonality, diapause, and photoperiodism in insects. The main ideas on this problem were reviewed by Lees (1955), Danilevsky (1961), Tauber et al. (1986), and Danks (1987). Mosquito diapause and its environmental control has been reviewed in monographs by Vinogradova (1969, 2000) and Mitchell (1988). The study of this topic in mosquitoes was additionally stimulated by the great applied importance of this taxa as active bloodsuckers and vectors of many agents