The common house fly, Musca domestica, and its relatives appear to vanish completely when the weather cools, returning in large numbers with the spring. This seasonal disappearance raises the question of whether cold weather simply kills them off. Fly survival through winter is a complex biological puzzle influenced by the species, the duration of the cold period, and the fly’s specific life stage. Cold temperatures are a significant source of mortality, but flies also possess strategies to survive conditions that are not immediately lethal.
The Critical Temperature Threshold
Cold weather kills flies through two primary mechanisms: immediate freezing and prolonged physiological failure, often called chill injury. For many common fly species, like the house fly, the true lower lethal temperature (LLT) causing immediate death by freezing is typically well below 0°C, often around -5°C or lower. Death at these sub-freezing temperatures occurs when ice crystals form within the fly’s body fluids, physically rupturing cells and tissues.
Mortality begins at much warmer temperatures. Temperatures significantly above freezing, such as 15°C, can cause low survival rates for house flies, especially if sustained. This is because a fly’s metabolism functions optimally within a narrow warm range. When temperatures drop but remain above freezing, the fly suffers from chill injury, involving a failure to maintain internal balance. This prolonged cold exposure leads to the loss of ion homeostasis, resulting in metabolic failure or dehydration.
Overwintering Through Diapause
When temperatures fall below the threshold for normal activity but remain above the lethal freezing point, many adult flies employ diapause. Diapause is a state of hormonally-induced developmental arrest and profound metabolic slowdown. Species like the face fly, Musca autumnalis, and cluster fly, Pollenia rudis, routinely use adult diapause to pass the winter months.
Physiological changes accompany this state, preparing the fly for long-term survival in cold conditions. A significant adaptation is the synthesis and accumulation of cryoprotectants within the hemolymph, or insect blood. These substances, including sugar alcohols like glycerol and trehalose, act as a biological antifreeze, lowering the freezing point of body fluids. Diapausing flies also seek sheltered microclimates, such as wall voids, attics, or under leaf litter, where temperatures remain consistently above the LLT.
Cold Exposure and the Fly Life Cycle
The cold weather’s impact is most pronounced on the immature life stages: the egg, larva (maggot), and pupa. Fly development is entirely temperature-dependent, meaning growth halts when temperatures fall below a certain point. The house fly life cycle, which takes seven to ten days in optimal warm conditions, can be extended to several weeks or months by cool weather.
Eggs and newly hatched larvae are particularly susceptible to cold stress. Very young house fly embryos, those just a few hours old, exhibit the lowest tolerance to chilling, with survival rates dropping significantly after short periods at temperatures like 5°C. Prolonged low temperatures can stall larval development, extending the maggot stage from a few days to eight weeks. Pupae, the stage where the fly transforms within a hardened shell, are often more cold-tolerant than the eggs but still exhibit reduced survival and delayed emergence at low temperatures. This slowing of the life cycle prevents reproduction during winter, causing the massive population reduction observed until the warmth returns.