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The advent of fall brings cooler weather, falling leaves, and new pest control challenges. As the temperature plummets and frost begins to coat the ground, some enterprising insect species opt to flee the outdoors for the more stable temperatures and protection from the elements available inside nearby homes and businesses. One such pest that's commonplace in York Region is the cluster fly.

During the spring and summer, female cluster flies will lay their eggs in the soil. Once the larvae hatch, they will burrow into the flesh of an earthworm, which they will use as a food source for the duration of their larval stage, lasting between two and three weeks. They then pupate, and emerge as adults after another two weeks of development. Cluster flies can breed between three and four new generations per year.

As temperatures begin to cool with the onset of fall, these flies can often be found in large numbers on the sunward facing walls of buildings. Cluster flies enter the home to overwinter, and will hibernate in low-traffic areas such as attics and wall voids. During colder months, they will occasionally awaken on sunny days, as the sun warms the voids in which they hibernate. When they're active, they will typically cluster in sunny windows.

Cluster flies are similar in appearance to the common house fly, about 8mm-1cm in length, with black or silver-black checkered bodies. Unlike the house fly, a cluster fly's wings overlap when the insect is at rest. These flies also tend to be slower in flight than house flies.

On their own, cluster flies are not destructive, and pose minimal health risks. They don't transmit disease, contaminate food, or damage the structure of the home. They breed out-of-doors rather than inside buildings, as their young are exclusively parasitic on earthworms. However, they are a distracting nuisance, and will leave droppings on the windows and window sills they're attracted to. They may also attract other pests, such as beetles and rodents, that feed on soft-bodied insects. Large numbers of cluster flies may cause an allergic reaction in those with sensitivities to them.

Cluster Fly Control

The best way to deal with cluster flies is to take preventative measures to deter them from entering your home or place of business to overwinter. Make sure your doors and windows are well-fitted, and don't have any holes in their screens. Caulk or fill any cracks in the building's exterior, and maintain your fascia and soffits in good condition. Make sure any vents, such as the exhaust vent for your dryer, are well-screened. It's difficult to eliminate all methods by which cluster flies can enter your home, as they're small enough to fit through minute openings, but by decreasing the number of avenues of entry, you decrease your risk of infestation.

If chemical prevention is necessary, you can contact a pest control professional to apply a pesticide treatment to your exterior walls, fascia, and soffits in the early fall to deter the flies from entering your building to overwinter.

If you have an active cluster fly infestation, it's recommended that you use a vacuum to eliminate individual flies rather than swatting them, as crushing them will release an unpleasant odour, and may result in stains on any surface they were resting on when they were swatted. If the infestation is severe enough that control of individual flies is not sufficient, it's recommended that you call in a pest control professional, who can help you determine where the cluster flies are hibernating and effectively treat the problem. Treatment requires fogging the attic spaces or wall voids where the flies overwinter, and applying a pesticide treatment to the window frames their drawn to in order to eliminate stragglers.