Many insects (& other arthropods) use freeze avoidance, a survival strategy that prevents their body fluids from freezing at temperatures well below 0°C. Water “normally” freezes at 0°C because the growth of ice crystals is “seeded” by the presence of particles or surfaces that help to orient the water molecules into crystals. Once a microscopic nucleus of ice is formed this triggers high speed crystal growth where the water molecules quickly join together. However, without such seeding, water can actually be cooled to much lower temperatures without ice formation – indeed, microscopic droplets of highly pure water can actually be cooled to ‑40°C before they freeze.
Freeze avoiding animals use this property of water and add specialized molecules to their body fluids to promote supercooling and prevent freezing even when temperatures drop well below the normal freezing point of their body fluids. Insects are experts at this and are aided by their small body size (the smaller something is, the better it is at supercooling) and their strong, waxy cuticle that coats their bodies to provide superb water/ice proofing. Indeed, even in summer, most insects will not freeze until at least -5°C or lower. In the winter, cold hardy species initiate adaptations that push supercooling even farther. Insects that spend the winter near the ground surface (e.g. on the forest floor or in the soil under your lawn) where they are covered by an insulating blanket of snow, can generally supercool to -15°C or -20°C without freezing. However, insects that spend the winter high up in the trees or in other exposed sites can often supercool to -40°C. The main adaptations that support the ability of insects to supercool and avoid freezing include: (1) Antifreeze proteins, (2) Cryoprotectants, and (3) Metabolic arrest.
(1) Antifreeze Proteins: These function by binding to microscopic ice crystals as they start to form and, in doing so, prevent crystals from growing any larger. Most insects that spend the winter in insulated places such as under a blanket of snow only have to add antifreeze proteins to their body fluids to get the protection that they need. Indeed, various insects and other invertebrates can actually stay active under the snow since temperatures rarely fall below -5 to -8°C despite air temperatures above the snowpack that could fall to -20°C to -30°C or lower.
(2) Cryoprotectants: High concentrations of polyols and sugars are synthesized to push the supercooling point of body fluids to low levels. The most common cryoprotectant used by insects is glycerol, a 3-carbon polyhydric alcohol that is the longer version of the 2-carbon ethylene glycol that we use to keep our car radiators from freezing. Several other polyols as well as some kinds of sugars are also used as cryoprotectants by some species, sorbitol being another common one. The freezing point of water decreases in proportion to the concentration of dissolved solutes in it, so the more cryoprotectant that is packed into body fluids, the lower the temperature at which an insect will freeze. The key advantages of glycerol as the most common cryoprotectant are that it is highly soluble in water, is not toxic to metabolism, and can be easily synthesized by the insect. The concentrations of glycerol can be so high in some insect species in midwinter that as much as 20-25% of the insect’s total body mass is glycerol. Glycerol is also very good at binding to water and, for insects that live in exposed sites such as in trees, this helps them avoid desiccation in the dry cold air of winter.
(3) Metabolic arrest and ischemia tolerance: Many insects use strong metabolic rate depression to lower their energy needs during the winter months. Often this is induced by hormones in response to changes in photoperiod and thermoperiod so that the last generation produced in the late summer or autumn enters a state of diapause (arrested development) that is sustained for several months until spring is near.
Freeze avoiding animals: Goldenrod gall moth