So long, see you next year

By Athayde Tonhasca

The woods are lovely, dark and deep,
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.
Robert Frost

When temperatures begin to fall, life becomes hard for insects. Some species try to escape the cold by sheltering in holes in the ground, under tree barks or even inside our homes; others, like bumble bees and some moths, generate internal heat by biochemical processes, just like we do. However, these strategies are not enough to protect such small creatures when it gets really cold.

© Malene Thyssen, Wikipedia Creative Commons

Many insects just leave: some flies and butterflies undertake journeys of several thousand kilometres to spend the winter around the Mediterranean, returning to the UK in spring. But most insects cannot escape. For those that stay put, the greatest winter threat is the formation of ice inside their bodies. Ice expands, tearing through body tissues. If you’ve ever put lettuce or apples in the freezer and let them thaw, you have seen the catastrophic effects of internal ice.

To avoid freezing to death, insects go into diapause, which is a state of dormancy similar to hibernation. This can happen at the egg, pupa, larva or adult stage, depending of the species. We may think diapause is prompted by temperature, but in fact the daily light regime (photoperiod) is the main trigger. Temperature alone is not a reliable yardstick; a cold spell may induce diapause too early, and a prolonged autumn may delay it. Either way, the wrong timing could be fatal.

When photoperiod reaches a threshold, a series of events kicks in. In the case of an adult insect, it begins accumulating fats, proteins and carbohydrates. Hormones and enzymes induce a thickening of the cuticle (‘skin’) to reduce water loss. The insect then will look for a cosy, safe hidden spot such as inside tree trunks, under rocks or underground. Breathing and metabolism drop to the bare minimum. The insect produces large amounts of cryoprotectants (cryo:  cold) such as glycerol, sorbitol and sugars, which act as antifreeze – similar to the products we use in our cars. In the same way that vodka remains liquid in the freezer, these compounds don’t freeze.

A German wasp queen (Vespula germanica) hibernating inside a fallen tree © MaxNikon, Wikipedia Creative Commons

By lying dormant, and topped up with natural antifreeze, insects have a good chance of surviving the winter. These adaptations are far from perfect; mortality is high, and often just a few individuals make it through. 

But the resilience of some insects is truly admirable. When temperatures drop, the flightless Antarctic midge (Belgica antarctica) produces large amounts of antifreeze sugars and loses up to 70% of its body water. It becomes so dehydrated it can’t freeze. The Arctic woolly bear moth (Gynaephora groenlandica) spends roughly 90% of its life in diapause as a caterpillar, and it survives temperatures as low as −70 °C. Predators and parasites may get at these insects, but a blizzard or two and freezing conditions are nothing. It’s adaptation at its best. 

An Arctic bear moth caterpillar © Barrio, I.C. et al. 2013. Arctic 66(4)