The familiar buzzing of flies often diminishes with colder weather, leading many to believe these insects simply vanish or perish. While their numbers decrease, flies employ various strategies to survive the winter months, ensuring their return when temperatures rise again.
How Flies Endure Cold
Flies utilize several physiological mechanisms to survive cold, with a primary strategy being diapause. This state of suspended development and reduced metabolic activity is triggered by environmental cues like decreasing temperatures and shorter day lengths. During diapause, a fly’s metabolism slows significantly, allowing it to conserve energy and remain dormant, often until spring. Some fly species enter diapause as adults, while others overwinter as larvae or pupae, with their development paused in these immature stages.
Many fly species also produce cryoprotectants, antifreeze-like compounds that help prevent their internal fluids from freezing. Glycerol is a common example, accumulating in the fly’s body to lower the freezing point of its cells. This allows the fly to withstand temperatures that would otherwise cause lethal ice crystal formation. The presence of glycerol can enhance the effectiveness of naturally occurring antifreeze proteins, providing a more robust defense against freezing.
Some flies also upregulate specific heat shock proteins (Hsps) during diapause, which protect cellular components from cold stress. Not all flies employ these internal antifreeze mechanisms. Certain species, like the face fly, are “cold-avoidant,” meaning they do not produce antifreeze compounds. Instead, they rely on finding warm, protected microclimates to escape freezing temperatures. These diverse adaptations allow different fly populations to persist through the winter.
Where Flies Overwinter
Flies seeking refuge from winter cold find shelter in various protected locations, both outdoors and indoors. Many outdoor species utilize natural environments that offer insulation and protection from extreme elements. Common outdoor overwintering sites include leaf litter, loose soil, under tree bark, inside tree holes, and large piles of decaying organic matter. These spots provide a stable microclimate, shielding flies from freezing winds and precipitation.
Buildings also serve as significant overwintering sites, drawing flies into human structures as temperatures drop. Flies can enter homes and other buildings through small cracks and openings around windows, doors, and utility lines. Once inside, they seek out secluded and undisturbed areas, such as attics, wall voids, basements, and garages. These indoor spaces offer consistent, warmer temperatures and protection from predators, creating an ideal environment for dormancy.
Why Flies Are Sometimes Seen Indoors
Despite the general disappearance of flies in winter, some species are frequently observed indoors. Cluster flies, for instance, are well-known for seeking buildings as overwintering sites. They typically enter structures in late summer or early fall, congregating in attics, wall voids, and other hidden spaces where they remain dormant.
Face flies also migrate indoors to overwinter as adults, often found in barns, sheds, and homes. On warmer, sunny days during winter, the slight increase in temperature can cause these dormant cluster and face flies to become temporarily active. They may then emerge, drawn to light, and wander into living areas, creating a nuisance.
Common house flies, while more abundant in warmer months, can persist indoors throughout winter if conditions are favorable. They are adept at finding warm, sheltered areas within homes that provide access to food and moisture. If a house fly finds sufficient food sources, like food scraps or garbage, and a warm environment, it can survive and sometimes even continue to breed indoors during the colder season.