When temperatures drop and food sources disappear, insects, which are cold-blooded creatures, face significant challenges in temperate and polar climates. Unlike mammals that generate internal heat, insects are ectotherms, meaning their body temperature is governed by the external environment. This dependency means that winter survival requires a complete overhaul of their physiology, behavior, and life cycle. Many insect species have evolved diverse survival strategies to cope with freezing temperatures, a lack of liquid water, and limited nutritional resources. These adaptations range from physically escaping the cold to chemically altering internal body fluids to prevent ice formation.
The Great Escape: Insect Migration
For a few insect species, the solution to winter is simply to leave the cold behind and relocate to warmer regions. This strategy, while spectacular, is statistically the least common method of overwintering employed by insects globally. The act of migration allows these insects to completely avoid the physiological stress of freezing temperatures and resource scarcity. The most famous example is the Monarch butterfly, which undertakes a multi-generational, thousands-of-mile journey from the United States and Canada to the oyamel fir forests in central Mexico. Other insects, like the Common Green Darner dragonfly and certain species of moths and leafhoppers, also migrate south. However, the individuals that return north in the spring are often the offspring or subsequent generations of the insects that migrated south the previous fall.
Pausing Life: Diapause and Dormancy
The most widespread strategy for surviving winter is entering diapause, a state of dormancy involving a genetically programmed suspension of development and metabolism. Diapause is distinct from simple quiescence, which is a temporary, immediate shutdown in response to cold that is immediately reversible. This state is often triggered by predictable environmental cues, such as the decreasing duration of daylight hours (photoperiod), rather than the immediate onset of cold.
Once triggered, diapause involves a profound metabolic reorganization, including the cessation of feeding and a significant drop in metabolic rate. Insects can enter this suspended state at any life stage, including as an egg, larva, pupa, or adult, depending on the species. For example, the Woolly Bear caterpillar overwinters as a larva, while the Mourning Cloak butterfly enters diapause as an adult, sheltered in tree cavities or under bark. This preparation allows the insect to accumulate energy reserves and synthesize protective compounds.
Chemical Warfare: Freeze Avoidance
To survive sub-zero temperatures, many insects rely on biochemical adaptations to prevent the formation of lethal ice crystals inside their cells. The two main physiological strategies are freeze avoidance and freeze tolerance. Freeze-avoiding insects cannot survive internal ice formation and prevent it by lowering the freezing point of their body fluids through supercooling.
They achieve this by producing high concentrations of small molecules known as cryoprotectants, such as glycerol or sorbitol, which act like biological antifreeze. These compounds colligatively depress the supercooling point, allowing the insect’s body fluids to remain liquid even at temperatures as low as \(-20\) degrees Celsius. Furthermore, these insects meticulously purge their gut contents to eliminate ice-nucleating agents—substances that might otherwise trigger freezing at warmer sub-zero temperatures. In contrast, freeze-tolerant insects, such as the Goldenrod Gall Fly larva, survive by controlling the freezing process, using ice-nucleating proteins to initiate freezing outside their cells, thereby protecting the delicate internal cellular structures.
Hiding Out: Shelter and Habitat Selection
Regardless of their physiological preparation, insects must select a physical location that provides a stable microclimate to buffer them from extreme temperature fluctuations. Most insects seek out microrefugia—small, protected areas where temperatures are significantly warmer than the ambient air.
Common overwintering sites include:
- Beneath the insulation of leaf litter
- Inside the soil below the frost line
- Under the loose bark of trees
- In brush piles or crevices
Many lady beetles and ground beetles seek shelter in these areas. Aquatic insects, like certain dragonfly and mayfly nymphs, descend to the bottom of ponds and streams to live under the ice. The insulating quality of deep snowpack also provides a stable environment, shielding insects from the most severe air temperatures and fatal freeze-thaw cycles.