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What are Cluster Flies?

When fall arrives, bugs seem to disappear from our lives until spring returns. However, some bugs like to stick around throughout the winter months. Cluster flies have a habit of moving into homes to avoid the chilly winter weather. We’ve gathered information about cluster flies to help you identify and eliminate an infestation.


Cluster fly

Cluster flies are similar in appearance to house flies, but slightly larger. They grow between 8-10mm in length. Cluster flies have a dark gray or black body with small, golden hairs on their thorax. A checkered pattern can be seen on a cluster fly’s abdomen, which helps separate them from other flies. These flies have a pair of translucent wings that overlap when resting. Like other flies, they have two large eyes.



When temperatures begin to drop, cluster flies gather on the exterior of homes and buildings. Toward the end of fall, these flies will try to move inside in search of a safe place to overwinter. They prefer areas that are dark and protected like wall voids and attics, which is why they are sometimes referred to as attic flies. Cluster flies move slower than other flies and can release a sweet odor when disturbed or crushed. While adult cluster flies feed on nectar from plants, their larvae feed on their earthworm host. Cluster flies don’t breed inside. Instead, they lay eggs in the soil where larvae can easily find earthworms.


 Cluster fly on cup

When cluster flies find a safe and warm location to overwinter, they release a pheromone that will attract other flies to the area. This results in large numbers of cluster flies gathering near or in your home. While they won’t bite humans, you don’t want to let these flies hang out in your home. Though they are inactive in the winter, they will begin to emerge when temperatures rise. Since they prefer hidden locations, eliminating them can be difficult. This also presents a problem if they die and can’t be removed from your home. Dead cluster flies attract beetles and carpenter ants. Their feces can also leave dark stains on walls and windows.

Warning Signs


Spotting cluster flies is the main warning sign of an infestation. They often gather on windows where there’s plenty of sunlight to keep them warm. You’ll likely see cluster flies on windows or the sunny side of your home before they try to move inside. If there are a few flies buzzing around inside, there’s a good chance others are hiding nearby. You may also notice a buzzing noise when there are multiple flies in a location. Small, black spots on windows or walls could be a sign of an infestation as well. Cluster fly excrement can stain walls, furniture, or fabric.


Sealing window with caulk gun

Keeping cluster flies out of your home begins with preventative measures. Like other bugs, cluster flies can slip into small spaces. This is why it’s important to check and seal any cracks or holes. Make sure to check near common entry points like doors, windows, and electrical outlets. If your window or door screens have any tears, you’ll want to repair them. Another option is to treat your yard, including the perimeter of your home. Using a spray that kills on contact and provides residual repellency, like Maggie’s Farm Simply Effective Yard Bug Spray, can help keep bugs out. The residual repellency acts as a barrier and deters pests from entering your home. September to mid-October is the best time to apply treatment.


Maggie’s Farm Simply Effective Home Bug SprayMaggie’s Farm Simply Effective Flying Insect Killer

Sometimes cluster flies ignore no vacancy signs and sneak inside homes. If this happens, there are things you can do to eliminate an infestation. Using a natural, botanical pesticide, like Maggie’s Farm Simply Effective Home Bug Spray or Maggie’s Farm Simply Effective Flying Insect Killer, is a more environmentally and family-friendly solution. Flies directly sprayed will be killed and the residual repellency will help keep cluster flies from returning. It’s important to sweep or vacuum any dead flies you find to avoid attracting other pests to your home. Make sure to discard them outside, away from your house.  

Cluster flies may want to hideout in your home, but that doesn’t mean you have to let them. Eliminating entry points and taking other preventative measures can help keep your home cluster fly-free. If you’re dealing with a pest problem, we want to help! Check out our Maggie’s Farm Simply Effective Pest Control products.

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