As summer fades and the chill of winter sets in, many of us breathe a sigh of relief from the persistent buzzing and bothersome presence of flies. From farms to waste disposal sites, and even within our homes, fly populations noticeably decrease as temperatures drop. This seasonal decline often leads to a common assumption: that flies simply die off during the winter months. But is this really the case? If flies were to completely disappear each winter, where would they come from when warmer weather returns? We’ve long since moved past the outdated notion of flies spontaneously generating from decaying matter, thanks to groundbreaking experiments like those of Francesco Redi in the 17th century.
So, if they aren’t spontaneously appearing and not entirely dying out, Where Do Flies Go During The Winter? The answer lies within the fascinating life cycle of these insects. Redi’s work, which disproved spontaneous generation by demonstrating that flies originate from eggs laid by other flies, actually provides a crucial clue. Understanding the juvenile stages of flies is key to unraveling their winter survival strategies. Flies undergo complete metamorphosis, a four-stage life cycle encompassing egg, larva, pupa, and adult. Many fly species are well-adapted to survive the cold winter months by overwintering in one of these immature stages. This means they don’t vanish completely; instead, they endure winter’s harsh conditions, ready to re-emerge when temperatures rise. While cluster flies (Pollenia rudis) are well-known for overwintering as adults, often becoming a nuisance in attics, numerous other fly species also have fascinating cold-weather survival techniques worth exploring.
House Flies (Musca domestica)
The common house fly, Musca domestica, is a ubiquitous pest, and its winter survival strategies are particularly intriguing. Research has revealed that house flies can successfully overwinter as adults in environments like refuse tips. These locations, seemingly inhospitable, often contain pockets of fermenting waste that generate and retain significant heat, creating surprisingly warm microclimates even in freezing external temperatures. One study observed house flies not only sheltering in these warm refuse pockets but also actively mating. Further investigation into these refuse sites uncovered all stages of house fly larvae, indicating continuous breeding throughout the winter in these insulated environments. Adult house flies were found sheltering as late as mid-January, and newly emerged adults were observed by the end of February, demonstrating the species’ ability to maintain year-round populations in temperate climates by exploiting these thermally stable microhabitats.
Farm environments also offer overwintering opportunities for Musca domestica. Observations on farms have shown that while adult house flies remain present in small numbers throughout winter, larvae are less commonly found in manure heaps, which can become quite cold. However, larvae have been discovered in animal bedding within cattle stalls where temperatures remain consistently above 20°C (68°F). These localized warm spots become crucial refuges for house flies, enabling them to survive and even continue development despite the surrounding winter chill. Laboratory experiments further support this, showing that low temperatures can significantly slow down house fly development, extending it up to 90 days. This physiological flexibility allows them to endure prolonged cold periods. Crucially, house fly overwintering sites need to maintain temperatures above -5°C (23°F) and provide enough periods above 10°C (50°F) to allow for egg, larval, and pupal development, highlighting the importance of these warm microclimates for their winter survival.
Stable Flies (Stomoxys calcitrans)
House flies aren’t the only fly species equipped for winter survival. The stable fly, Stomoxys calcitrans, often called the “biting house fly” due to its painful bite, also has effective overwintering strategies. Research has focused on how stable fly larvae respond to temperature changes within manure mounds, a common breeding site. Models simulating larval movement in response to temperature gradients have shown that stable fly larvae can readily avoid freezing temperatures by migrating downwards within manure mounds. This vertical movement allows them to access warmer, deeper layers of the manure, effectively escaping the freezing surface temperatures of winter. This behavioral adaptation is a key factor in stable fly overwintering success, allowing larvae to survive in manure heaps throughout the colder months.
Antarctic Midge (Belgica antarctica)
For an extreme example of cold-weather adaptation, consider Belgica antarctica, the Antarctic midge. This remarkable insect is the only six-legged species native to the continent of Antarctica, a land of extreme cold. Uniquely, it is wingless, an adaptation likely evolved to prevent being blown away in the strong Antarctic winds, ensuring survival in this harsh environment. The larval stage of Belgica antarctica exhibits extraordinary cold tolerance, capable of surviving being encased in ice for most of the year. This incredible resilience to freezing temperatures allows this fly species to thrive in one of the coldest places on Earth, showcasing the diverse ways flies have adapted to survive winter conditions, even in the most extreme environments.
Thaumatomyia notata
Thaumatomyia notata is a small fly, only about 2mm long, somewhat resembling fruit flies. Its larvae feed on plant roots, specifically consuming aphids of the genus Pemphigus. Adult flies, in contrast, feed on flower nectar. Notably, Thaumatomyia notata can form very large swarms, often seen around tall buildings with lawns. These swarms can be so dense they’ve been mistaken for smoke, even triggering false fire alarms. Thaumatomyia notata can complete 2-4 generations per year. While less directly related to winter survival, understanding their life cycle helps to see how different fly species adapt to various ecological niches and seasonal changes.
Face Flies (Musca autumnalis)
The face fly, or autumn housefly, Musca autumnalis, is easily confused with the common house fly Musca domestica. However, subtle differences exist: males can be distinguished by eye spacing, and females by abdominal coloring. Female face flies lay their eggs (30-230 at a time) individually on fresh cow pats, ideally less than an hour old. During summer, female face flies are commonly found on cattle and horses, feeding on eye and nasal fluids, saliva, and blood when available, needing a protein-rich diet for egg production. Males, conversely, primarily feed on flower nectar. Face flies become inactive during strong winds, heavy rain, and temperatures below 15°C (59°F). For overwintering, they favor the warmer south and west-facing sides of buildings, seeking out sun-exposed and sheltered locations to survive the winter months.
Conclusion
In conclusion, the notion that flies simply “die off” in winter is a misconception. Instead, flies employ a range of overwintering strategies to survive the cold. Many species overwinter as larvae or pupae, while others, like house flies and face flies, seek sheltered, warmer locations as adults, often “invading” buildings to escape the elements. Before seeking indoor shelter, these insects often congregate on sunlit building facades, maximizing warmth from sunlight.
As spring approaches and temperatures rise, these overwintering flies become active again, seeking to return outdoors. This resurgence can sometimes be an unpleasant surprise for homeowners and businesses if flies have successfully overwintered indoors. Even on warmer winter days, flies may become temporarily active, offering a preview of the larger emergence to come.
Preventive measures can significantly reduce fly invasion. Sealing building facades, fitting windows with insect screens, and using shutter seals on shutters are effective steps. For commercial settings, the Titan 300 by PestWest offers a powerful tool for managing fly infestations. It can handle heavy fly populations and can be adapted as a “cluster fly unit” by removing the collection tray. Beyond active infestations, the Titan 300 serves as a valuable preventive measure, helping to capture flies before they establish winter habitats indoors, contributing to long-term fly management.
