The Winter Disappearance of Flies
As colder weather arrives, flies largely disappear. Most adult flies cannot regulate their internal body temperature and perish with freezing temperatures. Their short lifespans mean many complete their life cycle before winter.
Food sources that sustain adult flies, such as decaying organic matter, nectar, and various secretions, become scarce or disappear entirely in winter. This lack of sustenance contributes to the decline of adult fly populations. The primary reason for their seasonal disappearance is their inability to withstand prolonged cold and the depletion of their food supply.
Survival Strategies of Different Fly Species
While many adult flies succumb to winter, various species employ biological mechanisms to survive colder months. These strategies allow their lineage to continue and re-emerge when conditions become favorable.
One common survival strategy among insects, including some flies, is diapause. This state of suspended development, similar to hibernation, is triggered by decreasing daylight and temperature. During diapause, flies significantly reduce their metabolic rate, conserving energy and allowing them to endure periods of cold and food scarcity. Some species, such as house flies and cluster flies, can enter this semi-dormant state as adults.
Not all flies survive winter as adults. Many species overwinter in more resilient life stages, such as eggs, larvae (maggots), or pupae. These immature stages are more tolerant to cold, protected within insulated environments like soil, decaying organic matter, or manure piles. House flies, for instance, commonly overwinter as larvae or pupae. Fruit flies, while sensitive to prolonged freezing, can survive as eggs, larvae, or pupae in sheltered environments or fallen fruit. Some insects also produce natural antifreezes, like glycerol, within their bodies to prevent ice crystal formation, further enhancing their cold tolerance.
Some adult flies actively seek protected environments to escape the cold. These include natural shelters like under tree bark, in leaf litter, or within hollow logs. Other species, such as cluster flies and face flies, seek human-made structures for overwintering. They aggregate in wall voids, attics, or basements, remaining dormant until warmer conditions return.
Why Flies Sometimes Appear Indoors in Winter
Flies sometimes appear indoors during winter due to specific overwintering behaviors. Certain fly species, particularly cluster flies and face flies, actively seek human structures as shelter from the cold.
Beginning in late summer or early autumn, these flies congregate on the sun-warmed sides of buildings, especially southern and western exposures. They then exploit small entry points, such as cracks around windows and doors, or unsealed vents, to gain access to wall voids, attics, and other secluded spaces within the structure. Once inside, they enter a dormant state, often forming loose clusters, which gives cluster flies their name.
These flies are not reproducing indoors; they are merely dormant. However, during warmer spells in winter or when indoor heating increases, the flies may become temporarily active. Attracted to light, they often emerge into living spaces, appearing near windows or light fixtures, creating a nuisance. This activity does not indicate a new infestation but rather the reawakening of flies that have been overwintering within the building.
The Reemergence of Flies in Spring
As winter recedes and temperatures begin to rise, flies that successfully survived the cold months initiate the next phase of their life cycle. For those species that overwintered as eggs, larvae, or pupae in protected environments, the increasing warmth signals the completion of their development. They emerge as adults, ready to begin reproducing.
Adult flies that entered diapause within sheltered locations, including those in wall voids or attics of buildings, also become active again with the warming weather. These flies will seek to exit structures and return to the outdoors, often being drawn to light sources like windows and doors as they try to escape. This reemergence marks the resumption of their life cycles, leading to a noticeable increase in fly populations as spring progresses.