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Some pests enter your home and may not cause any physical damage at all. This may seem like a relief, but these pests can be more stressful than any others. Cluster flies fall into this category. They are virtually harmless, but can gather in your home in the winter very large numbers and swarm around windows on warm days, trying to get back out. Getting rid of cluster flies is similar to other nuisance insects, but it is helpful to know more about them can help you to know how to get rid of cluster flies.
Cluster flies are larger than the common housefly with a dull gray color and black markings. These flies can have multiple generations per year. The adult cluster flies search out sunny areas in the autumn to overwinter. This can be sunny sides of houses, south facing rock faces or cliffs, crevices and voids in dead or dying tree bark, or other manmade structures. When spring arrives, the females lay their eggs in the soil. The larvae hatch and seek out earthworms, which are their food source.
Cluster flies are harmless, but certainly unwanted in any home. They do not cause physical damage to structures, do not reproduce indoors, and do not carry any diseases. They will, however, gather in your home in very large numbers when the weather gets cool, similarly to Asian lady beetles. They will hide in wall voids or other crevices or cracks, and will swarm out during warm winter days, trying to find a way outside and “clustering” on windows. These flies are sluggish and easily swatted, but their large numbers make them very irritating.
As with many nuisance pests, mechanical exclusion (using physical barriers) is the best way to keep cluster flies and all other pests out of your home. Sealing all cracks around windows, doors, siding, utility pipes, behind chimneys, and all possible entry points with silicone-latex caulk or vine mesh screens can help to keep them out. Exterior applications of insecticides containing pyrethroids prior to the autumn overwintering can help prevent flies from overwintering on your home or entering your house. If flies have already entered your home, they may be in wall voids or other out-of-sight places. If they come out, vacuuming and sticky traps can help to cut back on fly populations. You can choose a pesticide formulated for use indoors. Pick one with pyrethrins and always read instructions before application.