How Do Insects Survive Winter? Antifreeze, Diapause and Controlled Freezing

Cold itself is survivable; ice is not, at least not carelessly. When water freezes inside a cell, the growing crystals puncture membranes and the cell dies, and the damage is mechanical rather than a matter of temperature. There is a second and subtler problem, since as water freezes in the spaces between cells it leaves the remaining fluid more concentrated, which draws water out of the cells by osmosis and dehydrates them severely. Any strategy for surviving the season must therefore either prevent ice forming at all, control where it forms, or tolerate the consequences. On top of this sits the energy problem, since nothing is growing to eat and a small body has very little in reserve, which is why most overwintering insects shut their metabolism down to a fraction of its summer rate.

The central adaptation is not a response to cold but a programme entered in advance. Diapause is a hormonally controlled state of arrested development, triggered principally by shortening day length in late summer, which is a reliable signal in a way that temperature is not, since a warm week in October predicts nothing. Once entered, development stops, metabolism drops, feeding ceases, and the insect will not resume even if the weather turns warm, which prevents a fatal false start. Preparation during this window includes accumulating fat, emptying the gut of food particles and bacteria that could seed ice formation, losing water deliberately, and synthesising protective compounds. Each species has a fixed stage at which it does this: some overwinter only as eggs, some only as larvae, pupae or adults, which is why a particular insect can be found in only one form in January.

Species divide into those that avoid freezing and those that permit it, and the biochemistry differs completely:

• •Freeze-avoidant insects supercool, remaining liquid well below zero, some to minus forty, by clearing every particle that could act as a seed for a crystal and flooding the body with glycerol, sorbitol and trehalose, which lower the freezing point and stabilise membranes and proteins
• •They also produce antifreeze proteins, which do not lower the freezing point much but bind to any tiny crystal that forms and stop it growing, a mechanism first found in polar fish and since in many insects
• •Freeze-tolerant insects do the opposite, deliberately producing ice-nucleating proteins so that freezing begins early, slowly and outside the cells, where it does no mechanical damage
• •They accumulate the same protective sugars and alcohols to keep the cells from dehydrating as the extracellular fluid concentrates, and some tolerate more than half their body water frozen
• •The woolly bear caterpillar of the Arctic is the extreme case, freezing solid each winter and taking as many as fourteen years to accumulate enough summer feeding to pupate
• •A few species, including some midge larvae, survive by dehydrating almost completely, since a body with no free water cannot form ice

Chemistry is not the only answer, and many insects simply avoid the conditions. Migration is the most spectacular, with monarch butterflies travelling up to four thousand kilometres to a small area of Mexican mountain forest where they cluster in a state of near-dormancy, and several dragonfly and hoverfly species making comparable journeys. Others go underground, below the frost line, which is why soil-dwelling larvae are common. Aquatic insects overwinter under lake ice where the water stays at 4 degrees. Social insects use collective heat: honeybees do not hibernate at all but form a tight cluster and shiver their flight muscles, rotating between the warm centre and the cool exterior and holding the core near thirty degrees on stored honey all winter, which is why a colony that runs out of stores starves rather than freezes. Many species shelter in leaf litter, under bark or inside houses, which is where the ladybirds in the window frame come from.

The system depends on the timing of cues, and warming disrupts it in ways that are not simply helpful. Because diapause is triggered by day length and broken by accumulated warmth, mild winters can allow insects to end dormancy early and emerge before their food plants, a mismatch that starves them. Repeated freeze and thaw cycles are more damaging than steady cold, since each cycle costs energy and each thaw restarts metabolism, draining reserves that were calculated for one continuous winter. Snow cover matters more than air temperature for ground-dwelling species, because snow insulates, and a warmer winter with less snow can mean colder soil. Against that, milder winters have allowed several damaging species to extend their range northward, including the mountain pine beetle, whose spread across western North America was limited by extreme cold snaps that have become less frequent, with consequences visible across millions of hectares of dead forest.

Ice rather than cold is the danger, because crystals rupture cells and freezing outside them draws water out by osmosis. Insects prepare through diapause, a hormonally arrested state triggered by shortening days rather than by temperature, during which they empty the gut, lose water and accumulate glycerol and sugars. Freeze-avoidant species supercool to as low as minus forty with antifreeze proteins, freeze-tolerant species deliberately trigger ice outside their cells, and others migrate, burrow or, in the case of honeybees, cluster and shiver all winter.