Flies disappear in colder months but return with spring, raising questions about their survival in freezing temperatures. While many insects succumb to cold, flies use various strategies to persist through winter.
General Impact of Cold on Flies
Most fly species are not tolerant of freezing temperatures. When temperatures drop, their metabolism slows, leading to inactivity. Without protection, most adult flies perish in prolonged sub-zero conditions.
Flies are ectothermic, meaning their body temperature mirrors their surroundings. As temperatures fall, their internal physiological processes become less efficient. This slowdown can lead to chill coma, where they become immobile and appear lifeless, even if not yet frozen.
Strategies for Cold Survival
Some fly species have developed adaptations to survive cold. A primary strategy is diapause, a state of suspended development and reduced metabolic activity. This allows flies to conserve energy and endure harsh conditions by slowing bodily functions.
Beyond diapause, certain flies produce cryoprotectants, natural “antifreeze” compounds like glycerol and trehalose. Glycerol depresses the freezing point of bodily fluids, preventing damaging ice crystals. Antifreeze proteins (AFPs) bind to ice crystals, inhibiting their growth and protecting tissues.
Behavioral adaptations also play a role. As temperatures fall, flies seek sheltered locations. These include natural refuges like leaf litter, under tree bark, or in soil, which offer insulation. Many species, such as house flies and cluster flies, also seek warmth and protection inside human structures like attics, basements, or garages. Clustering in these spaces can further aid in conserving heat.
Life Cycle Stages and Cold Tolerance
Flies’ cold tolerance varies across their life stages: egg, larva, pupa, and adult. Adult flies are often most vulnerable to freezing temperatures and may die off during winter. However, some species overwinter primarily as eggs, larvae, or pupae, which are more resilient.
These immature stages can withstand colder conditions due to their smaller size, sheltered locations, and sometimes, higher cryoprotectant concentrations. For instance, blowfly eggs tolerate temperatures down to -25°C, while their larvae and pupae show moderate cold resistance, ranging from -7°C to -13°C. Development duration for these stages depends on temperature; colder conditions can significantly extend the time for a larva to mature into an adult, from days to weeks.
When Flies Reappear
Flies’ return in warmer weather links directly to their winter survival strategies. As temperatures rise and daylight hours increase, cues triggering diapause diminish, signaling the end of this dormant state. Surviving adult flies emerge from sheltered overwintering sites, and eggs, larvae, or pupae that endured the cold complete development.
Re-emergence timing varies by fly species and local climate. For example, cluster flies, which often overwinter as adults in diapause within buildings, may become active on unseasonably warm winter days. Consistent spring warmth allows for rapid development of new generations, leading to a noticeable increase in fly populations.