In late fall (usually September and October) I always hear from friends that suddenly have tons of flies buzzing sluggishly around their house. They didn’t smell anything obviously dead, and are frustrated by the constant (it seems) procession of flies collecting on their window sills.
Cluster Flies get their name from their habit of clustering in attics or near windows. Although they are blowfly relatives, these flies are not carrion flies, but a type of fly parasitic on earthworms. These flies are harmless, and do not carry any human diseases. Unless you have an earthworm farm inside your house, you don’t have to worry about the flies laying eggs on anything, or reproducing.
These flies are just looking for a warm place to spend the winter. You’ll see them clustering on the outside of a home during the fall, usually in a spot warmed by the sun. They’ll find shelter from the winter by crawling into the eaves or under the siding–and you can’t really stop them, since completely sealing your house is almost impossible. (You should try to seal all the big cracks for heating efficiency anyway, and this will help reduce the number of flies.)
If the flies happen to crawl close to the inside of your house, they’ll warm up when you turn on the heat and find a way in–via electrical outlets, baseboards, or an attic door. You can usually vacuum them up, since they aren’t that lively.
You’ll have a second sudden influx of flies in the spring, as the outside of the house begins to warm, and flies that were hiding under the siding, or in the attic, begin to wake up and wander around. Who needs the Swallows of Capistrano–we have the cluster flies to tell us it’s spring!
These flies don’t cause any home damage, although you will see “fly specks,” or small brown spots, where the flies poop. These come off with soap and water.
Mostly these flies are just free cat entertainment. (If your cat eats these flies, they won’t hurt her. protein!)
[Image via Cliffie ]