Please Read: This site contains information about using pesticides. Pesticides can be dangerous if used improperly or carelessly. Always follow label instructions. The pesticide product label is the final word on the safe and legal use of pesticides. Questions about pesticide toxicity can be explored at the National Pesticide Information Center (NPIC). Control suggestions are intended for users in the state of Oregon, USA. Others should consult with local experts before relying solely on this information.

Cluster flies 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 is about the size of a house fly, dark grey in color with golden hairs on its thorax, behind the head, and light patches on its abdomen. While cluster flies can be extremely annoying, they pose no threat what-so-ever in terms of damage to our structures or as a health concern. They do not spread disease. Cluster flies normally live outdoors. Adults feed on nectar while the larvae, or maggots, (and here is where it gets interesting) are internal parasites of earthworms!

The female fly lays her eggs in the soil. Upon hatching in a few days the larva locates 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 where they spend the winter months, then emerge the following summer to begin the cycle again.

There are no good control measures for cluster fly larvae. Problems 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. Since 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 repair screens. If flys get inside, "total release aerosol insecticides" (bombs) are effective. Follow label instructions carefully.

National Pesticide Information Center

Iowa State University Entomology Internet Resources

OSU Extension Service

OSU Extension & Experiment Station Communications (Publications)