As temperatures drop and winter approaches, flies seem to vanish almost entirely. This seasonal disappearance prompts questions about where these creatures go and how they survive the cold.
Survival Strategies: Enduring the Cold
Flies employ several strategies to endure cold temperatures, as they are cold-blooded and rely on external heat sources. One mechanism is diapause, a dormant state resembling hibernation. During diapause, a fly’s metabolism significantly slows, conserving energy in unfavorable conditions. This response is triggered by environmental cues like reduced day length and colder temperatures.
Flies also seek sheltered environments to escape the cold. They find refuge in cracks in buildings, attics, wall voids, or basements. Outdoors, they seek protection under leaf litter, in soil, inside rotting logs, or in compost heaps and manure piles which generate warmth. These locations provide insulation from freezing temperatures for survival.
Not all adult flies survive the winter. For many species, immature stages—eggs, larvae (maggots), or pupae—overwinter dormantly. House flies, for instance, overwinter as pupae or adults in refuse. Cluster flies often survive as adults in diapause within sheltered structures. Some species, like the stable fly, have larvae migrate deeper into manure mounds to avoid freezing.
The definition of “cold” is relative and varies among fly species, referring to temperatures below their optimal activity range. Housefly overwintering areas typically need to remain above -5°C, with enough time over 10°C to permit development. Some flies produce cryoprotectants like glycerol as a natural antifreeze. This helps prevent ice crystal formation, aiding cold tolerance.
The Return: Re-Emergence in Warmer Weather
As temperatures rise in spring, environmental signals trigger the end of diapause. This warming prompts dormant flies or immature stages to become active. Physiological changes reverse, and metabolic rates increase.
Overwintering eggs, larvae, or pupae complete development into adult flies. Housefly eggs, for example, hatch within 8 to 24 hours under ideal conditions. The entire life cycle from egg to adult takes as little as 7 to 10 days in warm temperatures. Fruit flies also exhibit accelerated development with rising temperatures; development from egg to adult takes about 8.5 days at 25°C, but over 50 days at 12°C.
Once conditions are favorable, flies reproduce rapidly. Females become active and lay eggs, with some species laying hundreds in their short lifespan. Shortened generational intervals in warmer weather mean that populations multiply exponentially. The first flies observed in spring are often survivors who emerged from diapause or the initial generation that hatched from overwintered eggs, larvae, or pupae. This rapid reproductive cycle ensures their noticeable reappearance.