Flies, like all insects, are ectotherms, meaning their body temperature is regulated by the environment. Since they cannot generate enough internal heat to remain active in freezing temperatures, flies must adopt strategies to survive the cold. Their disappearance in autumn is not a migration but a combination of physical sheltering and deep physiological change. This allows them to endure months of cold until spring returns, hinging on finding a suitable location and initiating a state of suspended animation.
The Biological Strategy: Entering Diapause
The primary biological mechanism flies use to survive winter is a genetically programmed state of dormancy known as diapause. This state is triggered preemptively by environmental cues, such as the shortening of daylight hours, rather than just a drop in temperature. During diapause, a fly’s metabolism slows significantly, conserving energy for long-term survival.
This metabolic suppression is accompanied by a biochemical transformation to enhance cold tolerance. The fly accumulates cryoprotectants, substances that function similarly to antifreeze. These compounds, such as glycerol, flood the fly’s body fluids. By increasing the concentration of these solutes, the fly lowers the freezing point of its internal water, which prevents the formation of damaging ice crystals.
The accumulation of cryoprotectants helps the insect avoid freezing, a strategy used by most fly species to survive sub-zero temperatures. Some cold-tolerant fly larvae can even survive deep freezing. Flies may also modify the composition of their cellular membranes, making them more flexible and resistant to cold damage.
The Physical Shelter: Common Overwintering Locations
Flies must find a physical location that provides thermal stability, as diapause only protects them down to a certain temperature threshold. Overwintering sites must buffer the insects from extreme temperature fluctuations. Natural outdoor sites include subterranean locations, where the earth’s insulating properties keep temperatures relatively constant.
Flies often burrow into soil, seek refuge within thick leaf litter, or hide deep within compost piles. These environments provide a stable microclimate, often warmer than the ambient air, protected from wind and direct exposure. Other natural shelters include the space underneath loose tree bark, hollows of dead logs, or rock crevices.
Many species near human habitation utilize man-made structures for winter refuge. They are attracted to the warmth and stability of buildings, often squeezing into attics, basements, and wall voids. These indoor locations provide an ideal environment for dormancy, offering protection from low temperatures and desiccation. The flies remain dormant until the warmth of spring.
Varying Survival Stages: Adult vs. Larval Overwintering
The specific strategy a fly uses to overwinter depends on its species and the stage of its life cycle when cold weather arrives. Flies employ either adult dormancy or survival in an immature stage.
Immature Stage Survival
Certain species, like the common house fly, do not survive the winter as adults in cold climates. Instead, the adults die off, and the species survives as eggs, larvae, or pupae. These immature stages enter diapause while buried within their food sources, such as deep manure piles or compost. When warmer weather returns, these dormant life stages complete their development, leading to the emergence of a new generation of adults in the spring.
Adult Dormancy
Other species, most notably the cluster fly, survive the winter as fully formed adults. These adults enter diapause and actively seek out sheltered locations, often congregating in large numbers inside wall voids and attics of buildings. This behavior, known as “clustering,” helps them conserve energy throughout the winter. These adult-overwintering flies emerge ready to reproduce when temperatures consistently rise in the spring.