The presence of flies often becomes noticeable with the arrival of warmer weather, marking the beginning of what is commonly referred to as “fly season.” This period signifies a significant increase in fly activity and populations. The changes in temperature and environmental conditions during this time create a favorable environment for flies to thrive and reproduce.
General Timing of Fly Season
Fly season typically begins in temperate climates with the onset of spring, as temperatures start to rise. While the exact timing can vary regionally based on specific climate patterns, flies generally become active when conditions are sufficiently warm for them to emerge, feed, and breed. In many areas, this can mean a noticeable increase in fly populations as early as mid-March, continuing through April and May.
The emergence of flies is not a sudden event but rather a gradual increase in activity as environmental conditions become more suitable. In the Northeast United States, fly activity often starts in April, peaking during the summer months. Similarly, the West Coast, with its milder weather, experiences fly activity for most of the year, with peak numbers from April to October. Midwestern regions typically observe flies emerging in May and remaining active until the first frost, usually in October or early November.
Environmental Triggers
The initiation and continuation of fly season are closely tied to specific environmental conditions, primarily temperature, humidity, and the availability of food sources. Rising temperatures play a crucial role, accelerating the metabolic rate of flies and speeding up their life cycle stages, including egg hatching, larval development, and pupation. For many fly species, optimal breeding temperatures range from approximately 75°F to 85°F (24°C to 29°C), with development time significantly decreasing under these conditions.
Humidity also contributes to the favorable conditions for fly reproduction and survival. Flies generally thrive in warm, moist, and high-humidity environments. Increased humidity can make it easier for flies to remain active and reproduce more frequently, as moisture is essential for their hydration and development. Egg hatching success and adult emergence can increase with rising relative humidities, while developmental time may decrease.
The availability of decaying organic matter is another significant trigger, as it serves as both a food source and a breeding ground. Female flies deposit their eggs on various forms of decaying organic matter, such as excrement, rotting food, garbage, plant matter, and carrion. Larvae consume this decaying material for their maturation. Different fly species have preferences; for example, house flies often lay eggs on feces or rotting food, while blow flies prefer carrion. This abundance of suitable breeding sites and food sources during warmer, humid periods directly contributes to the rapid increase in fly populations.
How Long Does Fly Season Last?
Fly season extends throughout the summer months and gradually tapers off into early autumn. After the initial increase in spring, fly populations typically reach their peak in late summer. This high activity continues as long as temperatures remain conducive for breeding and survival. As the weather transitions into fall, and temperatures begin to drop, flies become less active.
The breeding season for many flies generally comes to an end around October. Fly activity significantly decreases when temperatures fall below approximately 45 degrees Fahrenheit (7 degrees Celsius). During these colder months, many flies enter a pupal stage, often buried in the soil, or seek shelter in protected areas like attics or wall voids to overwinter in a hibernation-like state called diapause, where their metabolism slows. However, some species, like cluster flies, may become active again on warmer winter days.