In much of Canada and the US, people are either competing for "late date" observations of hardy species such as Autumn Meadowhawk (Sympetrum vicinum), or whiling away the cold, dark, dragonfly-less days of winter perusing photos and field guides. The onset of wintry weather can also make people wonder just how dragonflies and damselflies, which are largely tropical in their origins, manage to survive in cold climates.
Insects have evolved many methods to deal with the cold. Some take refuge in numbers, such as honeybees that cluster together in hives, vibrating their wings and eating stored honey; others hibernate as adults, like Mourning Cloak (Nymphalis antiopa) butterflies and lady beetles. But adult dragonflies can't survive a cold winter--so what's an odonate to do when things get frosty? In some species, adults avoid cold weather by flying south. The Wandering Glider (Pantala flavescens), for example, for all its wide distribution, is essentially a tropical species that cannot survive winters above about the 40th parallel, e.g., the Nebraska/Kansas border. This species breeds in temporary pools where the nymphs develop quickly to avoid habitat drying, and the adults that emerge move on to new sites.
In others, adults die with the onset of winter but the species persists as cold-tolerant eggs or nymphs. A combination of changes in day length (photoperiod) and temperature triggers an arrest in development, and the insect enters a resting stage called diapause. Egg-stage diapause occurs in some Aeshna (darners), Lestes (pond spreadwings), and Sympetrum (meadowhawks), but nymph-stage diapause is more commonly seen among temperate zone dragonflies. Some nymphs can survive temperatures as low as -5C (23F), but their tissues don't freeze solid due to a variety of freeze tolerance mechanisms, which may include finding warmer microhabitats and/or adjusting the levels of certain alcohols or sugars in their blood to lower their freezing point.
Some species use a combination of strategies to ensure dispersal and survival under adverse or changing conditions. The Common Green Darner (Anax junius), our best-known migrant in North America, exhibits both adult migration and nymphal diapause. Many of you saw large numbers of these beautiful dragons zooming southwards in fall, fleeing the shortening days and colder nights of their natal wetlands for warmer southern waters, where the eggs they lay develop rapidly into the adults that return to the north in the spring. But Common Green Darners can also remain in northern habitats as nymphs, surviving the winter in diapause and resuming development after the waters have warmed the following spring, emerging as adults weeks after their migrant cousins have returned and laid eggs.
Scientists studying this phenomenon in the late 1960's thought it unlikely that Common Green Darner nymphs could overwinter as far north as Montreal (45.5N), but by the early 2000s, nymphs were known to overwinter successfully at this latitude, suggesting potential effects of global climate change. Impacts on odonate phenology, distribution, and community structure due to climate change have already been noted, and forecasts of the effects of rising temperatures include shorter generation times and northward range expansions, which may ultimately affect the timing and occurrence of migration. However, the details of seasonal regulation in odonates have only been studied for a few species, making data reported through projects such as Migration Monitoring and Pond Watch even more important for our understanding of these complex and beautiful insects.