Medium-sized flies from 1/4-inch to 3/8-inch in length.
A number of insects have discovered that heated buildings are ideal for surviving the cold of winter. Such pests include box elder bugs, cluster flies, and ladybugs. As the weather cools in late summer and early fall, the sun warms the southern and western walls of buildings. The warmth attracts these insects to buildings where they crawl inside cracks and stay there for the winter. This would be fine, but during warm winter days, some insects “wake up” and end up on the inside of the building.
Cluster flies are actually parasites of earthworms. The adult flies deposit their eggs in the soil and the larvae seek out and live off of earthworms.
The best way to control cluster flies is by prevention as described below. If it’s too late and they are already inside, it takes a professional to find and treat the right areas to minimize the numbers of pests seen inside. If your home has experienced a problem with overwintering pests, take the following steps next summer to prevent a recurrence: Seal as many cracks and holes on the outside of the home as possible, especially on the south and west walls where the sun heats the home during the late summer and fall. Be sure that all foundation and attic vents have tight-fitting screens. Check the soffit vents and any gable vents or turbine vents on the roof. Have your Terminix professional treat the outside west and south walls of the home near the eaves. This treatment should be completed in mid- to late August.