This is the time of year when various outside insects try to get inside to spend the winter. One such invader is the cluster fly. Cluster flies look a lot like house flies, but they are slightly larger and slightly slower-moving than house flies. Other clues are that cluster flies’ wings overlap over their backs when at rest, and they have golden hairs covering their thorax (the part between the head and the abdomen).

Where Do Cluster Flies Come From?

Cluster flies are interesting (at least to an entomologist) because their larval stage is parasitic on earthworms. The larvae develop underground inside the earthworm’s body, then pupate, and emerge as adult flies. Not every home will play host to cluster flies, and no one fully understands why they end up where they do. Suburban homes with large lawns and rural homes near fields tend to have cluster flies more often. The shape of your home and the construction of the roof may be factors, too.

In late summer, cluster flies can be seen resting on the sunny sides of buildings. They tend to move upward on a building and will enter through openings, primarily around the roofline. They can often be found in large numbers in the attic. Cluster flies become very sluggish when inside and may not be seen except on sunny, warm winter days when they emerge and head for the light at windows (see Flies Indoors in Winter?). In spring, cluster flies will “wake up” and try to find their way back outside. At this time, they can usually be easily swatted or vacuumed.

What Can You Do About Indoor Cluster Flies?

Once cluster flies are inside, control is difficult as they hide in obscure places. Large numbers in attics or wall or ceiling voids can be treated with insecticides. To be effective, cluster fly control must be initiated in late summer or early fall as the flies are starting to move inside (see Control Cluster Flies Before They Get Inside). At that time, treating the exterior of buildings can kill the flies as they look for openings. Pest-proofing or sealing those openings will also help to keep them out.

Give Colonial Pest a call. If you’ve noticed cluster flies in your home, we may be able to help by eliminating flies that are on the move or that are clustered together in voids. More importantly, we can talk to you about setting up a complete fly management program for next year that includes outside sprays and pest-proofing measures.