Flies disappear due to the fundamental shift in environmental conditions that occurs during the transition from autumn to winter. As the days shorten and temperatures decline, the insect world receives two clear signals that the favorable growing season is ending. This seasonal change triggers a widespread and rapid collapse in the number of active adult flies. This decline is a predictable, biological response to reduced daylight and the onset of cold, forcing the species to shift into survival mode rather than continuous reproduction.
The Seasonal Trigger: When Fly Populations Crash
The widespread disappearance of flies is directly tied to the temperature falling below the threshold required for successful development and sustained activity. The general population of flies, including common house flies, begins to slow down significantly when average daily temperatures drop below approximately 13°C (55°F). Below this point, their metabolic rate slows, and the time it takes for eggs to develop into adults increases dramatically, effectively halting the reproductive cycle.
A sustained period where temperatures fall further, particularly below 5°C (41°F), is lethal for most active adult flies. Their movement ceases, and they cannot regulate their body temperature. This widespread mortality is the primary reason for the noticeable population crash in late autumn. Reduced photoperiod (the shortening of daylight hours) acts as a secondary, predictive signal, informing the insects that winter is approaching and initiating the physiological changes necessary for survival.
Overwintering Strategies: Where Do Flies Go?
While the adult population dies off, the fly species survives the winter through a state of arrested development known as diapause. Diapause is a pre-programmed dormancy, distinct from simple inactivity caused by cold, and is triggered by environmental cues like the shortening photoperiod. This allows the insect to prepare biochemically for the cold, often accumulating cryoprotectants like polyhydric alcohols to lower their internal freezing point.
Different fly species enter diapause at different life stages. For example, some blow flies and flesh flies overwinter as a non-feeding, third-instar larva or a pupa, sheltered within the soil or protected organic matter. The house fly, which tends to have greater cold tolerance, often survives as a larva or pupa buried deep within large masses of fermenting material, such as compost piles or manure. The decomposition process generates enough internal heat in these locations to prevent freezing. This strategic choice allows the species to re-emerge rapidly once spring temperatures return.
Why Some Flies Linger Indoors
The occasional presence of sluggish flies indoors during the winter is a result of a specific overwintering behavior, not a sign of outdoor survival. Certain species, most notably the Cluster Fly (Pollenia rudis), actively seek out human structures in the autumn to enter diapause and escape the lethal cold. As outdoor temperatures drop, these flies aggregate on the sun-warmed, south and west-facing exterior walls of buildings.
They then crawl into cracks, crevices, and voids around window frames, eaves, and vents to find a dry, secluded place to spend the winter. Once inside, the flies become dormant, often gathering in large, inactive clusters, which is why they are sometimes called attic flies. If the weather outside warms up unexpectedly, or if the heat from the structure’s living spaces penetrates the void, some flies may prematurely emerge into the home. They appear confused and slow-moving as they attempt to locate a way back outside.