Cold weather kills many insects, but the survival of countless others demonstrates a remarkable array of defense mechanisms. Mortality depends on the specific species, the stage of its life cycle, and the severity and duration of the environmental conditions. Insects that survive the winter employ one of two primary strategies: they either tolerate freezing temperatures through internal chemical changes or avoid the cold altogether through behavioral and migratory tactics.
When Cold Temperatures Become Lethal
For many insects, death occurs immediately upon reaching a specific temperature threshold, known as the Lower Lethal Temperature (LLT). This often aligns with the Supercooling Point (SCP), the temperature at which water in the insect’s body fluids spontaneously freezes below the usual freezing point.
Most insects are freeze-intolerant; the formation of even a single ice crystal inside their body is fatal. Ice crystallization causes lethal mechanical damage, rupturing cell membranes. Furthermore, as extracellular water turns to ice, it draws water out of the cells, resulting in osmotic shock. Once the SCP is breached, death is rapid due to the uncontrolled formation of ice.
A small number of species are freeze-tolerant, possessing the ability to survive with ice crystals present in their extracellular fluid. These insects use specialized proteins, known as ice-nucleating agents, to control the freezing process. These agents initiate freezing at a relatively high sub-zero temperature (typically -5°C to -10°C), preventing the destructive, rapid freezing that occurs at much lower temperatures. Initiating ice formation in extracellular spaces reduces the risk of lethal intracellular freezing, allowing cells to manage the resulting dehydration.
Internal Chemistry: How Bugs Resist Freezing
Insects that survive winter fundamentally alter their internal composition to withstand low temperatures. The most significant adaptation for freeze-intolerant species is the massive production of cryoprotectants, which function like biological antifreeze. These compounds, including polyols such as glycerol and sorbitol, are synthesized from the insect’s stored glycogen reserves as temperatures drop.
The cryoprotectants work in two primary ways to prevent freezing. They exert a colligative effect by dissolving in the body fluid, substantially lowering the freezing point of the insect’s hemolymph. Additionally, these molecules stabilize proteins and cellular membranes, helping them resist damage from concentrated salt solutions that form during cold exposure. The concentration of these chemicals can be high enough that the body fluids remain liquid far below the normal freezing point of water.
This internal chemical change is often coupled with diapause, a hormonally regulated dormancy that suspends development. Diapause drastically lowers the insect’s metabolic rate, conserving energy needed for cryoprotectant synthesis. To enhance cold hardiness, many species also practice strict water management, voiding the gut of food and waste. This eliminates potential ice-nucleating particles that could trigger lethal freezing at a warmer temperature.
Overwintering: Finding Shelter to Avoid the Chill
Beyond internal chemistry, insects rely on behavioral adaptations to find microclimates that buffer them from the weather. One effective natural shelter is the subnivean zone, the protected layer between the ground and the insulating snowpack. Once snow reaches about 15 centimeters (6 inches) deep, heat radiating from the soil keeps this layer remarkably stable, often hovering near 0°C (32°F) regardless of the frigid air temperature above.
Other insects seek refuge by burrowing or hiding in naturally insulated locations. Larvae and pupae, such as the corn rootworm, survive by burrowing deep into the soil, often below the frost line. Adult insects, like the Mourning Cloak butterfly, crawl into tree bark crevices or beneath loose logs. Many moth caterpillars and beetle larvae overwinter in the moisture-retaining insulation of thick leaf litter, a strategy used by the Great Spangled Fritillary caterpillar.
Some species use social behavior to generate and conserve warmth. Honeybee colonies survive by forming a tight cluster and generating heat through synchronized muscle contractions. Lady beetles often aggregate in massive clusters under leaf piles or rocks, reducing individual exposure. Other insects, like the Monarch butterfly, employ the ultimate avoidance tactic by migrating up to 3,000 miles to the temperate oyamel fir forests of central Mexico, escaping the lethal cold of northern habitats.