House flies are common insects, often a nuisance during warmer months. Their activity shifts with temperature, leading to questions about their whereabouts and survival strategies.
When Flies Begin to Disappear
Flies become less prevalent as outdoor temperatures decline, typically in late fall and early winter. Their activity is heavily influenced by ambient temperature, as they are cold-blooded creatures.
House flies, for instance, become inactive below 45 degrees Fahrenheit (7.2 degrees Celsius) and generally perish below 32 degrees Fahrenheit (0 degrees Celsius). Some species, like the autumn housefly, become inactive below 59 degrees Fahrenheit (15 degrees Celsius).
The timing varies significantly by geographical location and climate. In mild winter regions, some species remain active, breeding and foraging. However, in areas with distinct cold seasons, the visible adult fly population diminishes as temperatures fall.
The Science Behind Their Retreat
The decline in fly activity during colder periods is directly linked to their physiology as ectotherms. As temperatures drop, their metabolic rate slows considerably, impacting their ability to move, fly efficiently, and perform other biological functions. They become sluggish and eventually unable to fly at all.
Cold conditions also affect their reproductive cycles. While flies thrive and reproduce rapidly in warm environments, their egg-laying and development slow down or cease in colder temperatures. For example, house fly eggs hatching in less than eight hours at 99 degrees Fahrenheit (37.2 degrees Celsius) could take two days at 59 degrees Fahrenheit (15 degrees Celsius). Sustained low temperatures inhibit their ability to complete their life cycle effectively outdoors.
How Flies Survive Colder Months
While many adult flies succumb to the cold, various species employ strategies to survive winter. One common method is seeking shelter in protected indoor or outdoor locations like attics, basements, wall voids, leaf litter, or soil. Some species, like cluster flies, specifically seek out buildings in large numbers to overwinter.
Many fly species enter a state of dormancy called diapause, a period of suspended development and reduced metabolic activity. This conserves energy, allowing them to endure harsh conditions until temperatures rise again. Unlike true hibernation, diapause is a pre-programmed developmental arrest occurring at any life stage (egg, larval, or pupal). Many species overwinter as immature stages in insulated environments like manure piles or protected soil, emerging as adults in spring. Some flies can even produce a natural antifreeze, like glycerol, in their bodies to tolerate freezing temperatures by preventing ice crystal formation.