I was up in our attic this morning to check for leaks and the place was full of house flies! There must have been 200 to 300 of them. They were bumbling around more than flying. Why would they be in the attic. Should I fog them? T. Y., Somersworth, NH

They might have been house flies but it’s more likely at this time of year, that what you saw were overwintering cluster flies. Cluster flies look a lot like houseflies and they are known to be winter and spring pests inside homes (see Flies Indoors in Winter?).

How can you tell if they’re cluster flies? Cluster flies are slightly larger than a house fly and hold their wings overlapping over their backs when at rest, rather than out to the sides like house flies. Cluster flies also do not have the 4 stripes on their backs that you see on house flies. You can also call us to have an expert inspect and identify your flies.

Cluster Flies Spend the Winter as Your Guests!

In late summer, cluster flies begin looking for protected places to spend the winter. They seek the warmth of nearby buildings and will find openings around the roofline, often ending up in the attic. Once they move into a building, they remain fairly inactive until spring or until warm, sunny days in winter get them going again.

In spring, cluster flies that have spent the winter in attics or wall voids “wake up” and start looking for ways out. They sometimes find their way into living areas first, startling residents. They’re attracted to light so they often end up buzzing around windows.

Not every home is visited by cluster flies in the fall. Because the larvae live in the ground where they parasitize earthworms, cluster flies are more abundant in areas with large open expanses of pasture or lawn. Suburban homes with large lawns and rural homes near fields are more likely to have overwintering cluster flies. You will probably have to deal with them moving in again next fall, unless you have a preventive exterior insecticide treatment.

Our Scheduled Treatment Can Keep Cluster Flies Out

You could call Colonial for a treatment of the flies in your attic, although by the time you call your flies may be gone. The more effective way to control cluster flies is to treat in late summer as they are congregating on buildings (see Control Cluster Flies Before They Get Inside). We have a Preventative Maintenance Program that is timed to intercept cluster flies and other overwintering insects that are looking to make your house their home. See Our Package Plan Keeps Pests Away Year Round.

We also offer pest exclusion measures that can be completed at any time of year. Caulking, screening, and sealing openings, especially around the roofline, will help keep overwintering insects out. Give Colonial a call today!