Each spring and fall cluster flies are a problem. They enter houses, or other shelters, in the fall. The flies tend to "cluster" in large numbers at windows, ceiling or other high places.
This fly resembles a house fly, is about the same size and similar in appearance. It's dark gray with golden hairs on its thorax and behind the head. Light-colored patches form a checkerboard pattern on its abdomen.
Cluster flies normally live outdoors. Their biology, however, is quite remarkable. Adults feed on nectar while the larvae (maggots) -- and here is where it gets interesting -- are internal parasites of earthworms!
The female fly lays her eggs in the soil. The larvae hatch in a few days, locate an earthworm, penetrates its body, and develops there. As many as four generations develop each summer. If earthworms are abundant, a substantial fly population can build. In the fall, adult flies seek shelter for the winter months, then emerge the following summer to begin the cycle again.
While cluster flies can be extremely annoying when they are indoors, they pose no threat what-so-ever in terms of damage to our structures or as a health concern. They don't spread disease.
There are no good control measures for cluster fly larvae. They are common around well cared for lawns that support a healthy earthworm population. They can also be a nuisance in buildings surrounded by rich pasture land and turf. But, because earthworms are generally considered beneficial, we don't want to do anything to discourage them.
The best course of action is to prevent entry of flies indoors by sealing around windows and doors, and repairing screens. If flies get indoors, "total release aerosol insecticides" (bombs) can be effective. Follow label instructions carefully.
