Insects are ectothermic meaning they don’t generate their own body heat, and rely on external sources to regulate their body temperature. Freezing temperatures are life threatening to many insects, so how do they survive winter?
One July morning I went down to my vegetable garden to see how the plants had survived the deep freeze (-3°C) of the night before. I wasn’t surprised to find my brassicas were covered in ice crystals, but I didn’t expect to find a cabbage white butterfly (Pieris rapae) larva also encrusted in ice. I decided to keep an eye on it, so after the day warmed up a bit I revisited the garden to find the caterpillar happily munching on the plant. How did it do it?
Cabbage white butterfly is an introduced species from Europe, and is well adapted to cold weather. Their larvae can easily survive a bit of frost but if very cold weather persisted it would kill them – so they avoid it by pupating. Cabbage white butterfly pupae can survive temperatures as low as -20°C! The pupae go into a type of hibernation, where further development is suspended, known as diapause. It is this dormancy strategy that allows many insects to survive unfavourable environmental conditions such as prolonged cold weather. Cabbage whites emerge from their winter sleep quite early and are one of the first butterflies to be seen in spring. But why don’t the pupae freeze solid at -20°C? Cabbage white butterfly is one of many insects whose cells manufacture antifreeze proteins which lower the freezing point of their body fluids by about 6°C. So as long as those freezing temperatures don’t persist the pupae will survive.
Many native and introduced butterflies and moths survive winter as diapausing pupae. Overwintering pupae are often buried under leaf litter or soil and may not require the antifreeze proteins. The adult butterflies and moths from these diapausing pupae emerge at various times during spring and early summer depending on the species.
Some ladybird species are known to form aggregations in winter or summer in a semi-dormant torpor. These dormancies can increase the ladybirds’ life span to about 2 years, which is quite long for an adult insect. People often wonder where all the ladybirds are when their favourite plants are being attacked by aphids in early spring. The aphids are usually from Europe and are more cold tolerant, so frosty mornings don’t bother them, whereas the ladybirds prefer warmer weather. Eventually the ladybirds will emerge and take care of the aphids, or another predator or parasite may get there first.
There are usually no adult dragonflies and damselflies to be seen during winter in many parts of Australia. They avoid cold weather by staying underwater as eggs or, more usually, larvae.
Termites are especially vulnerable to temperature change but termite colonies are well protected inside their mounds. The temperature inside a termite mound is very stable and may fluctuate as little as ±1°C throughout the year. Honey bees also keep warm in their hives during winter, feeding on stored honey and pollen, and emerging to forage on sunny days. Other insects like borers in timber and insects inside woody galls are well protected from cold temperatures, as are ants in their nests.
Australian native budworm (Helicoverpa punctigera) moths migrate considerable distances to find suitable winter food for their larvae. They usually spend winter in breeding populations in inland Australia, a period where there is good growth for larvae to feed on. That growth begins to dry off in spring, so the generation of moths which emerge in early spring migrate towards the coast where they breed through spring, summer and early autumn. These are the buggers that chew holes in your tomatoes and lots of other plants! Winter is usually a time of lean pickings on the coast for these caterpillars, so when the autumn generation of moths emerge they fly inland before the onset of winter.
So even though you may not see a lot of insects flying around during winter they may be hibernating nearby, or hidden away as pupae, or are poised to return from elsewhere when the weather warms up. Winter does not “kill off” insects, it just slows them down a bit.