Flies, often seen during warmer months, seem to vanish in winter, leading many to believe they simply die off. This overlooks the strategies these insects employ to survive colder temperatures. While some adult flies perish with frost, various species have evolved adaptations that allow them to persist through winter, ensuring their return in spring.
Surviving the Cold
Many fly species survive cold temperatures by entering diapause, a state of suspended development and reduced metabolic activity. This dormant state is triggered by environmental cues like decreasing day length and falling temperatures, allowing flies to conserve energy and withstand harsh conditions. During diapause, some flies produce “antifreeze” compounds like glycerol, which lowers the freezing point of their bodily fluids. This prevents ice crystal formation within their cells, enabling them to endure sub-zero temperatures.
Flies also undergo cold-hardening, accumulating fat reserves during the fall to provide energy throughout their dormant period. Their metabolism slows significantly, reducing the need for food and minimizing activity. This allows them to remain largely inactive for extended periods without succumbing to starvation. Behavioral adaptations, such as seeking sheltered locations, protect them from extreme cold and predators.
Where They Overwinter
The locations where flies overwinter vary by species and life stage. Many adult flies, particularly nuisance invaders like cluster flies, seek refuge inside buildings as temperatures drop, gathering in attics, wall voids, basements, and sheds. These sheltered indoor spaces provide stable temperatures and insulation from the elements. House flies can also overwinter as adults in warmer microhabitats like refuse tips or animal bedding, where fermenting organic matter generates heat.
Other fly species survive winter in different life stages, such as eggs, larvae (maggots), or pupae. These immature stages often overwinter in protected outdoor environments like under leaf litter, within the soil, or in large piles of organic material such as manure. House flies commonly survive as larvae or pupae in manure piles, which offer both insulation and a food source.
Emergence in Spring
As winter recedes and temperatures begin to rise, warming conditions signal the end of diapause for overwintering flies. This increase in temperature and longer daylight hours triggers a resumption of their metabolic activity and development. Flies that overwintered as eggs, larvae, or pupae complete their development, transforming into adult flies.
The timing of emergence varies depending on the species and regional climate, with activity increasing from late spring through early fall in most temperate areas. Once emerged, adult flies begin seeking food sources and initiating their reproductive cycles. Accelerated development in warmer spring environments allows for rapid population growth, leading to increased fly numbers as the season progresses.