So we’ve finally seen a bit of frost and temperatures have plummeted to below zero overnight and are hovering just above freezing during the day. Crisp sunny days in winter are very nice. Apart from the stunning sight of a frosty landscape, one benefit of the cold is that all the creepy crawlies seem to die. Or do they? The answer to that – some do if it’s really cold but most don’t. Insects create a feeling of revolt in most of us but we can only admire them for their domination of the planet and they do that with some amazing survival techniques. Spiders, we all know, are not insects, but I will include them in the term for the purposes of this blog as the strategies they use are at one with the rest of the insect population.
Problems facing invertebrates over the winter are lack of food and protection against the cold weather. Most trees and shrubs lose their leaves in winter and twigs produce little sap. There is effectively no food source if you are a ladybird, ant or grasshopper. In order to survive the winter months they hibernate. The Winter Moth and December Moth are exceptions to this rule as are the Linyphiid spiders (money spiders), although even these will die if temperatures drop really low. Both moths are able to fly at near zero temperatures and the Linyphiid spiders can make a web below freezing.
To avoid the cold weather many invertebrates burrow down into soil or leaf litter where it is much warmer than on the surface and safe from hungry birds and cold winds. In areas where it snows regularly, the snow acts as a blanket against the worst of the winter weather. Beneath the bark of trees is another favourite hiding place.
Some insects use special body fluids to protect themselves against the cold.
Freeze tolerant invertebrates such as the woolly bear, flightless midge and alpine cockroach are the ones that can survive being frozen solid. These pests can control where, when and to what extent ice crystals form within their bodies. The ice crystals don’t damage the cells and organs of the insect and when warmer temperatures return they become active again.
Freeze intolerant invertebrates are those that use special “anti-freeze” chemicals to stop themselves freezing. The insects we encounter here in the UK are more likely to be freeze intolerant than freeze tolerant, which is reserved for really cold climates. As such freeze intolerant insects can become “supercooled”. Supercooling is when liquid is cooled to below its freezing point and does not freeze. Out of interest freeze intolerant insects will die if the temperature drops below their supercooling point (although freeze tolerant ones still won’t). The beetle (pytho deplaratus) is both freeze tolerant and freeze intolerant. It can supercool to -54°C. If the temperature falls below that it can still survive because it is freeze tolerant.
Invertebrates that live in water fair rather better through our relatively mild winters. Water is a great insulator and because it loses heat less rapidly than land, stays more constant. As the most that can usually happen is a thin layer of surface ice on ponds, food below the surface remains plentiful, contributed to by the vast amount of leaves that fall off the trees each autumn. Aquatic invertebrates enjoy a time of winter growth thanks to the food reserves and also because colder water absorbs more oxygen from the air. In places where it gets very cold and ponds and streams freeze extensively inverterates suffer the same fate as those on land.
Whatever strategy the insect world uses, its world domination is legendary and the success of its techniques is apparent every summer when the place is full of them again.