As temperatures drop in the fall, some insects become less active, which is a relief. However, those same cooler temps make cluster flies come out in full force.
Slightly larger than the common house fly, cluster flies are dull-gray in color with black markings. They also have golden-yellow hairs on the thorax and they make their debut in autumn. In search of sunny sides of homes to escape the cold, the over-wintering pests crawl out of wall voids and attics to enjoy the warmth, clustering in large numbers on the sunny exterior of buildings, as well as at windows within the home. Note that these flies give off an odor that some describe as smelling like buckwheat honey.
Cluster flies may be a nuisance, but they won’t reproduce within your structure and do not carry any diseases. In fact, the only damage you’ll likely see are small dark-colored spots of excrement on windows and walls. (Gross.)
Many flies, like cluster flies, love the sunlight and warmth, which is often why they buzz against windows. So always make sure your open windows have screens to keep flies from entering. Likewise, don’t leave doors without screens open for long periods of time. Here’s more on how to get rid of flies, in general.
While you’ll find that cluster flies are slow enough to capture and swat, you might be hoping to avoid them altogether. Your best bet is to do a little investigation. Where are they coming from? If they’re on the exterior of your home, you can use exterior insecticides consisting of a synthetic pyrethroid or a neonicotinoid, which you’ll want to hire a licensed pest control professional to apply. It’s important to note, however, that because the application is occurring in a sunny area, which breaks down the insecticide, the effectiveness may decrease after a week.
Are the flies in your home? Locate the openings and seal those cracks! They’ll likely be behind baseboards, around windows and nearby door trim. You can also use a hand vacuum to get rid of them.
Other options include hanging sticky fly strips or using a glass jar with sweetened water inside and a perforated lid to trap them. Just be sure you make the holes large enough for the flies to enter and change out the water every day. Keeping food covered is also important. Cluster flies especially love decomposing food and sweets! A pyrethrum spray like can help, as well. Spray it lightly, as needed, for kill.
To prepare for the next season, you can spray areas where you suspect these pesky flies will cluster. In early summer, be sure all your suspect cracks are sealed up. You can also apply a residual pyrethroid-based insecticide such as or on the exterior of the building in late August or early September, right before adult cluster flies show up.
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