Q: In the past few years we have had an increasing problem of dead cluster flies accumulating in one top floor bedroom on the southwest side of our condo. This room has a skylight but no windows otherwise. Starting in late summer through the fall and winter, hundreds of dead flies accumulate in that room. We plan to have an exterminator come in this summer before “fly season” starts. What else can we do?
— Chris Sullivan, Akron
A: Cluster flies get their name because they cluster on the sunny sides of buildings in later summer or fall and crawl inside when they find a crack or other small opening, looking for a warm place to spend the winter. They congregate in attics and wall cavities when it’s cold and leave those spaces on warm days, trying to find a way back outside.
There’s not much you can do about the flies that are already in your home. Fortunately, they don’t cause damage or lay eggs indoors, and they’re not known to transmit diseases. Vacuuming is a good way to get rid of dead and sluggish flies.
Your best course of action is to prevent more flies from getting in. Caulk cracks and holes on the outside of the building, particularly in siding and along eaves, and especially on south- and west-facing walls. Make sure window and vent screens are in good repair and have small enough openings to keep flies out. Indoors, caulking cracks along baseboards and around electrical outlets, switches, light fixtures and window and door moldings can help trap flies inside walls.
You can also spray the outside walls — particularly the south- and west-facing walls — with a persistent insecticide before the flies try to get in. Because timing is critical and the products that work best aren’t easy for homeowners to obtain or apply, it may be a job best left to a pest control professional.
More information on controlling cluster flies is available from the Penn State Extension (http://tinyurl.com/clusterpenn), University of Wisconsin Extension (http://tinyurl.com/clusteruw) and Michigan State University (http://tinyurl.com/ culstermsu).
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