The house fly, Musca domestica, is a common pest that seems to vanish when the temperature drops, leading many to assume they perish. These insects, which originated in central Asia, have spread globally by associating closely with human habitation. House flies do not migrate to warmer climates. Instead, they employ a biological survival strategy to persist through the cold months, entering a state of profound dormancy called suspended animation. This allows them to endure the lack of food and low temperatures until spring returns.
The Biological Strategy of Diapause
The primary mechanism house flies use to survive winter is diapause, a state of developmental arrest. This is not simply hibernation, but a physiological shift triggered well in advance of the cold. The main environmental cue for inducing this change is the decreasing photoperiod, or the shortening of daylight hours, which reliably signals the onset of autumn.
Lowering temperatures also contribute, integrating with the photoperiod to modulate the intensity of the dormant state. Once triggered, diapause severely reduces the fly’s metabolic rate and stops growth and development. The fly’s body also undergoes a chemical transformation, accumulating cryoprotectants such as glycerol. This increases cold tolerance and prevents lethal ice crystal formation within its tissues. This preparation ensures the fly is ready to survive conditions that would otherwise be fatal.
Specific Overwintering Locations
To survive the winter, house flies must find a microhabitat that provides consistent thermal protection. Outdoors, they seek sites where decaying organic matter generates warmth, such as deep within manure piles, compost heaps, or refuse tips. Fermenting waste can maintain an internal temperature far above the ambient air, sometimes reaching 18°C to 25°C, allowing flies to survive or even breed during milder periods.
Indoors, flies seek shelter inside structures, relying on insulation and stability. They crawl into cracks, crevices, wall voids, and unheated spaces like attics and crawl spaces. These hiding spots offer a consistent temperature that remains above the critical freezing point. Successful overwintering requires a microclimate that typically stays above -5°C.
Flies entering buildings in the fall are often drawn to the sunny, warm sides of the structure, where they locate small entry points. While many perish, those that find a protected void remain inactive until a sudden warm spell or the arrival of spring reactivates them. Finding a secure, stable location is necessary for surviving the season and ensuring the population can rebound.
Which Life Stage Survives the Cold?
The house fly undergoes complete metamorphosis, passing through four stages: egg, larva, pupa, and adult. The species relies primarily on its immature stages to survive the cold, overwintering specifically as a larva or pupa. The larval stage, or maggot, burrows into a protected food source like a manure pile before entering diapause.
The pupal stage is resilient, encased in a tough shell called a puparium. This protective armor allows the developing fly inside to withstand harsher conditions than the exposed adult. While some adult flies may survive the winter in warm, sheltered indoor environments, they are short-lived. The population increase observed in the spring is mostly due to the emergence of new adult flies from the overwintered pupae and larvae.