We’ve been told that the flies that show up in our house every spring are cluster flies. How are these different from house flies and what can we do about them?

Cluster flies are similar in size, shape, and coloration to house flies. Two differences are that cluster flies hold their wings overlapping over their back when at rest, rather than out to the sides like house flies, and cluster flies do not have 4 stripes on their back like house flies.

Cluster flies have very different habits than house flies. Instead of feeding in decaying matter and garbage like house fly maggots, cluster fly larvae develop in the soil where they are internal parasites of earthworms!

It will no doubt come as a surprise that the flies you see in the spring aren’t entering your house but have actually been living in your house all winter! In late summer, adult cluster flies outside seek the warmth of buildings and try to move inside, looking for a sheltered place to spend the winter. Most homeowners don’t even notice that gradual migration. Once inside, the flies end up in attics, wall and ceiling voids, behind baseboards, around ceiling fans or lights, and in other hidden places. Buildings that are near pastures or large lawns have the biggest problem with cluster flies. You could end up with hundreds of flies spending the winter in your attic or wall voids and not even realize it. In spring or sometimes on warm, sunny days in winter, these overwintering flies become active and bumble about rather sluggishly, trying to find a way back outside. Because they are attracted to light, they often end up buzzing around windows.

If you have a small number of cluster flies, you can easily swat them or vacuum them or just scoot them back outside. They move slowly and they don’t bite. To keep them from entering in the fall, you or a pest control professional should check for and seal any openings into your home around the roofline, eaves, under siding, around doors and windows, conduit openings, vents, etc. Caulking, screening, and sealing openings can help keep cluster flies out.

If you’re one of those who has a major problem with overwintering cluster flies, you’ll probably want to have a professional treat around the outside of your home in the fall, at the time that the flies are starting to move inside. Call Colonial. We can evaluate your fly problem, inspect for overwintering flies, and help determine how they are entering. We can even treat your attic or other sites where the flies are hiding And we can set up a fall treatment program that will keep this summer’s flies from getting in to spend the coming winter.

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