The cluster fly (Pollenia rudis), commonly known as the common cluster fly, attic fly, or buckwheat fly, is a familiar nuisance for homeowners in North America, particularly during the fall and winter months. These flies are not harmful to humans or property, but their presence can be overwhelming when they enter homes in large numbers. Understanding how to identify and manage these pests is crucial for keeping them out of your living spaces.
Identification of Cluster Flies
Cluster flies are slightly larger than the common housefly (Musca domestica), measuring between 8 to 10 mm in length. They have a dull-gray appearance with black markings and distinctive golden-yellow hairs on their thorax, giving them a slight sheen. A key feature that sets them apart from houseflies is their slower flight pattern and the way their wings overlap when at rest, unlike houseflies that spread their wings apart.
The immature stages of the cluster fly, including the egg and larva, are rarely seen. The eggs are deposited in the soil, and the larvae burrow into earthworms, on which they feed. The larvae are cream-colored and shaped like a wedge. As members of the Calliphoridae family, also known as blow flies, cluster fly larvae resemble other blowfly larvae typically found on decaying matter, though they parasitize earthworms.
Where Cluster Flies Are Commonly Found
In North America, cluster flies are found throughout the continent, often becoming a problem in homes, barns, and other man-made structures during the colder months. They seek out warm, sheltered spots to hibernate during the fall and winter. Common hiding places include attics, wall voids, and other secluded areas within a building. These flies are attracted to the warmth of the sun, often gathering on the sunny sides of homes during the fall before finding their way inside.
Once inside, they tend to hibernate in clusters and remain dormant until warmer temperatures in late winter or early spring cause them to become active. On warm days, they often emerge from their hiding places, crawling toward windows in a confused attempt to return outdoors. They can be found in large numbers on window sills or near light fixtures.
Life Cycle of Cluster Flies
The life cycle of the cluster fly revolves around its unique relationship with earthworms, which serve as hosts for the larvae. In the spring and summer, adult cluster flies lay their eggs in the soil near earthworm burrows. Once the eggs hatch, the larvae seek out earthworms, burrow into them, and feed on the worm’s internal tissue. After about two weeks of feeding, the larvae pupate in the soil, emerging as adults to continue the cycle. Depending on the climate, cluster flies can produce up to four generations each year.
As the weather cools in the fall, adult cluster flies from the final generation seek shelter for the winter. This is when they begin to congregate on the sunny exteriors of buildings and eventually find their way inside. Cluster flies are not breeding inside homes; they are merely using these spaces to survive the winter before returning outdoors in the spring.
How to Prevent and Control Cluster Flies
One of the most effective methods for controlling cluster flies is to prevent them from entering your home in the first place. Here are some key strategies for managing cluster flies:
Mechanical Exclusion
Sealing entry points is the first line of defense against cluster flies. Inspect your home for cracks and gaps around windows, doors, siding, utility pipes, and chimneys. These should be sealed with a high-quality silicone or silicone-latex caulk. Damaged screens on windows and doors should also be repaired or replaced. Additionally, attic vents can be covered with fine mesh screens to prevent flies from entering these areas.
Exterior Insecticide Application
In some cases, exterior applications of insecticides can help reduce the number of flies entering a building. Insecticides containing synthetic pyrethroids or neonicotinoids can be applied by licensed pest control professionals to the exterior of a home in the early fall before the flies begin congregating. However, it’s important to note that the effectiveness of these treatments is limited because insecticides break down quickly when exposed to sunlight.
Vacuuming and Physical Removal
Once inside, cluster flies are relatively sluggish, making them easy to capture or vacuum. Vacuuming is one of the safest and most effective methods for removing cluster flies that have gathered in windows or other areas. Be sure to empty the vacuum bag outdoors to prevent the flies from re-entering the home.
Traps and Sticky Strips
For small infestations, sticky fly strips can be used in areas where flies are gathering. These strips are an inexpensive way to trap flies that are sluggish and moving about slowly.
Life Inside the Home
During the winter months, cluster flies tend to gather in attics, wall voids, and other concealed areas. While they do not reproduce indoors, they may leave small dark spots of excrement on windows and walls. Although they do not carry diseases, the sheer number of flies can create a significant nuisance for homeowners.
It is important to avoid using insecticides inside wall voids or attics where cluster flies have already gained access. Doing so may lead to additional pest problems, such as carpet beetles, which can feed on the dead flies and then cause damage to stored items like woolens or other natural products.
Where Cluster Flies Occur in North America
Cluster flies are widespread across most regions of North America, with the exception of areas near the Mexican border. They are particularly common in regions with temperate climates where earthworm populations thrive, as the larvae depend on earthworms for their development. The flies are most likely to become a problem in rural areas or in homes surrounded by large lawns or fields where earthworms are abundant.
Summary of Control Measures
To effectively manage cluster fly infestations:
- Seal entry points such as cracks and gaps around doors and windows.
- Apply exterior insecticides before the flies begin to enter, if necessary.
- Use a vacuum or fly strips to physically remove flies that have made their way indoors.
- Avoid insecticides inside wall voids, as they can lead to secondary infestations by other pests.
By taking these preventative measures and managing infestations early, homeowners can greatly reduce the likelihood of dealing with large swarms of cluster flies during the colder months.