Flies are a common seasonal nuisance whose activity is driven by environmental conditions, primarily temperature and light, rather than the calendar. The life cycles of common species like house flies and cluster flies shift significantly from year to year based on these factors. Understanding the specific factors that trigger their emergence and accelerate their reproduction provides a clearer picture of when and why they become a problem.
The Environmental Triggers for Fly Activity
The timing of fly emergence is directly linked to sustained warmth, which dictates when they can shift from dormancy to active reproduction. House flies typically begin to emerge from overwintering sites once outdoor temperatures consistently rise above 50°F to 60°F. This threshold temperature allows the adult flies to become mobile and seek out the decaying organic matter needed for egg-laying. Cluster flies, which often overwinter as adults inside structures, become active when outdoor temperatures reach the mid-50s or when indoor spaces are heated to that level.
Increasing daylight hours, or photoperiod, serves as a secondary signal, preparing flies for the active season. The combination of lengthening days and rising temperatures causes flies to leave their protected winter locations. While house flies tend to emerge quickly to begin breeding in garbage and manure, cluster flies often become noticeable indoors first, stirred from attics and wall voids by the increasing warmth of the sun on the structure.
How Temperature Accelerates the Fly Life Cycle
The reproductive speed of flies is dependent on ambient temperature, which is why a population can boom rapidly during a heatwave. Flies undergo complete metamorphosis, progressing through four distinct stages: egg, larva (maggot), pupa, and adult. The time it takes to complete this cycle is drastically compressed by heat, determining the severity of “fly season.”
For a house fly, the journey from egg to reproductive adult can take up to 45 days when temperatures hover around 61°F (16°C). However, if the temperature rises to 86°F (30°C), that same life cycle can be completed in as little as 10 days, allowing for exponential population growth. The larval stage, where the fly exists as a maggot, is particularly sensitive to heat, and what might take several weeks in cool conditions can be over in just three to seven days in optimal warmth. This accelerated development means that multiple generations can be produced in a single warm season, leading to the peak fly populations experienced during summer months.
Where Do Flies Go in the Winter?
When temperatures drop in the fall, flies do not simply perish; instead, they employ various survival strategies to endure the cold season. Many species enter a state of suspended animation called diapause, where their metabolism slows significantly to conserve energy. This is often triggered by both decreasing temperatures and the shortening photoperiod.
Specific species utilize different sheltered locations for this dormancy. Cluster flies, for instance, are known to seek protection inside buildings, congregating within secluded spaces like attics, wall voids, and basements. Other species, including house flies, often overwinter as pupae or larvae buried in protected organic matter, such as compost piles, leaf litter, or manure, emerging only when the warmth of spring returns. Occasional sightings of sluggish flies indoors during the winter are typically cluster flies that have been temporarily awakened from their diapause by a spike in indoor heating or a particularly warm, sunny day.