Editor’s Note: This article was reprinted with permission from Techletter, a biweekly training letter for professional pest control technicians from Pinto & Associates.

The cluster fly, Pollenia spp. (Family Calliphoridae), is a fall-invading fly that is a well-known seasonal pest. It’s found throughout Canada and the United States, except for states bordering the Gulf of Mexico.

Cluster flies are slightly larger and thinner than house flies at 3/8 of an inch. They are a dull gray (not metallic like some other Calliphoridae) with golden hairs on the thorax and a gray checkerboard pattern on the abdomen. When at rest, their wings usually overlap over their backs. When crushed, cluster flies are said to smell like buckwheat honey!

These flies have a unique life cycle. The larvae live in soil where they are internal parasites of earthworms. Consequently, structures with large lawns or those near fields are more likely to have problems with overwintering cluster flies. There are four generations a year but only the late season adults are considered pests when they move inside in late summer or fall. Adult cluster flies feed on flowers and are not attracted to garbage, feces or dead animals.

WHAT TO LOOK FOR. When days shorten, cluster flies look for protected places to spend the winter and often end up in structures. Cluster flies congregate first on a sunny side of a building. They usually enter through openings around the roofline, windows, vents or under siding and will quietly spend the winter in clusters in attics, wall voids or crevices.

Cluster flies are slow-moving flies, especially when they “wake up” indoors on warm winter days or in early spring as they look for a way outside. They head for light and often end up clustered around windows or in ceiling light fixtures.

Cluster flies should be controlled by creating barriers in mid-to-late-summer to prevent their entry: treatment of the exterior with a repellent residual, as well as pest-proofing of openings, especially around the roofline. It’s critical to time the residual application before fly entry in your area.

Once flies are inside, control is difficult and results are often disappointing. Avoid killing cluster flies in inaccessible voids because the dead flies can attract dermestid beetles. Vacuuming, sticky traps, light traps and space sprays can remove wandering flies.

WHAT TO REMEMBER. Cluster flies are seasonal overwintering pests that can become active on warm winter days. They do not reproduce indoors nor do they cause any damage. The best control is preventing their entry before their seasonal migration.

The authors are well-known industry consultants and co-owners of Pinto & Associates.