When the weather turns cold, the sudden disappearance of insects might suggest they prefer warmer climates. While these organisms cannot regulate their internal temperature like mammals, they have evolved sophisticated physiological and behavioral adaptations to survive winter conditions. Many species enter a state of suspended animation or employ complex biological “antifreeze” to endure sub-freezing temperatures rather than dying off entirely.
Internal Strategies for Cold Survival
Insects rely on two primary biochemical strategies to cope with low temperatures: freezing avoidance or freezing tolerance. The majority of species utilize freezing avoidance, which involves preventing the formation of ice crystals within their bodies entirely through a process known as supercooling.
To facilitate supercooling, insects synthesize large amounts of cryoprotectant compounds, such as the sugar alcohol glycerol, which act like biological antifreeze. These compounds increase the concentration of solutes in the insect’s hemolymph, depressing the supercooling point. Freezing-avoidant insects also actively eliminate ice nucleators, such as particles in their gut, which could otherwise trigger freezing prematurely.
A smaller group of insects employs freezing tolerance, meaning they can survive ice forming within their extracellular tissues. These species manage and control the process by producing ice-nucleating agents (INAs) that prompt freezing at a relatively high, controlled temperature.
Initiating the freeze at a warmer temperature allows ice crystals to form safely in the extracellular spaces, drawing water out of the cells. This controlled dehydration prevents the destructive formation of ice inside the cell membranes. Freeze-tolerant species still accumulate cryoprotectants, but their role is to protect the cells from damage due to extreme dehydration, rather than preventing the initial ice formation.
Seeking Refuge and Dormancy
Beyond internal chemistry, insects employ behavioral and metabolic shifts to match their environment’s demands. The most common strategy is diapause, a genetically programmed state of arrested development and metabolism distinct from simple hibernation. Diapause is typically triggered by predictive environmental cues, such as the shortening of daylight hours, causing the insect to prepare months before the cold arrives.
The monarch butterfly provides a well-known example of a behavioral refuge strategy, as the eastern North American population undertakes a multi-generational migration to overwinter in the oyamel fir forests of central Mexico. These mountain forests provide a microclimate with temperatures near freezing but with high humidity, which is necessary to prevent the butterflies from drying out.
Other insects, such as the convergent ladybug, utilize aggregation, gathering in massive groups. These dense clusters overwinter in protected spots like leaf litter, under bark, or high in mountain crevices. Aggregation helps to reduce moisture loss and offers collective protection against temperature fluctuations.
Many species seek out micro-refuges close to their feeding grounds, such as burrowing beneath the frost line in the soil, hiding under tree bark, or overwintering as eggs attached to twigs. Larvae of species like the Japanese beetle burrow deep into the soil where ground temperatures remain more stable. This combination of reduced metabolic activity and strategic location allows them to conserve energy reserves.
When Cold Becomes Lethal
Despite their adaptations, insect survival strategies have specific failure points that determine mortality during winter. The most straightforward cause of death is when the temperature drops below a species’ lower lethal temperature (LLT), the point at which internal defense mechanisms fail. For a freeze-avoidant insect, this occurs if the temperature falls below its supercooling point, leading to rapid, destructive internal ice formation.
A major threat is the instability of winter weather, specifically repeated freeze-thaw cycles. When temperatures fluctuate, insects can be prematurely roused from diapause, causing them to deplete their stored energy reserves. A return to deep cold after this energy expenditure leaves the insect vulnerable and unable to survive the remainder of the season.
Mortality is also high when insects fail to secure adequate shelter or when their chosen refuge is compromised. Furthermore, desiccation, or water loss, can be a significant killer during cold, dry winter periods, especially for species that do not aggregate to conserve moisture.