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Fall nuisance flies

Valerie Cervenka and Jeffrey Hahn

Several species of flies can be seen in and around homes during late summer and fall as they seek sheltered sites to overwinter. They are attracted to warm, sunny locations on the south- and west-facing sides in the upper half of buildings. Some of these flies crawl into cracks around siding, windows, vents and chimneys, and accidentally enter homes. They are a nuisance, and don't harm people or property.

In other cases, these flies end up in attics, wall voids and other spaces, where they hibernate when it becomes cold. On sunny, mild winter days, some flies may come out of their hiding places and crawl toward the warmth of the interior. They move inside through light fixtures, window pulley holes, moldings and baseboards. Although flies may be seen periodically throughout the winter, they do not reproduce indoors; all flies seen at this time entered the home last fall. When spring arrives, these flies emerge from their overwintering sites. Some successfully find their way outside, while others are trapped indoors and die.

Large numbers of dead flies in the walls of homes may be a source of food for larder beetles. Larder beetles are 1/4- to 3/8- inch long, oval in shape and are black with a yellowish-brown band across the body. The larvae are 3/8- to 5/8- inch long, brown, and covered with long hairs. Both adult and larval beetles feed on dead birds, animals and insects, and can infest homes where large numbers of overwintering flies have died indoors. Controlling cluster flies and face flies as they arrive in the fall helps reduce numbers of these beetles.

Cluster fly, Pollenia rudis

Cluster fly

Cluster fly (about three times actual size).
Photo by Jeff Hahn.

Cluster flies are named for their habit of overwintering in large clusters within the attics or upper walls of homes and buildings. They are black and a little larger than house flies, from 3/8- to 1/2-inch long, with short, yellow hairs on the thorax. Their wings overlap when at rest.

Cluster flies are unique because they are parasites of earthworms. In summer they lay eggs in cracks in the soil, and maggots enter earthworm hosts through the body wall. There are two or three generations of flies produced each year. In August and September, the final generation of flies seeks shelter for the winter in homes and buildings. Piles of dead cluster flies are often discovered in late spring when returning to northern vacation homes unoccupied during months when the flies emerge.

Face fly, Musca autumnalis

Face flies are closely related to and look very similar to house flies, but differ in their habits. Face flies are found only in pastures during summer, and their larval development sites are confined to fresh cattle dung pats. During the fall, adults fly to structures seeking sheltered hibernation sites, especially in the upper stories of buildings. They can be found in large numbers, often with cluster flies, in attics, steeples and little-used rooms.

Blow fly, Family Calliphoridae

Blow fly

Blow fly (about three times
actual size). Photo by Jeff Hahn.

Blow flies also can overwinter in buildings and in homes, but are not as common as cluster flies and face flies. They do not have a distinct clustering behavior, so few are found indoors during winter or early spring.

Fall nuisance fly control

The most effective method for fall nuisance fly control begins outside. Prevent adults from entering buildings by carefully sealing as many potential points of entry as possible. Entry points include gaps in siding and under eaves, vents to the outside, and cable and meter attachments. It is important to remember that, while providing some relief, caulking alone will not solve the problem. Caulking may be supplemented with a residual insecticide application in late summer or early fall. The window of opportunity for spraying is fairly narrow, from late August to mid-September in the northern half of Minnesota, and the first two weeks of September in the south. Spray the outside of the home or building with permethrin to reduce the number of flies trying to enter. A professional pest control operator can also be hired to apply an insecticide.

Flies found inside in the fall may be removed with a vacuum or killed with a flyswatter. Since flies are drawn to light, attract them to a bright window in an otherwise darkened room, making it easier to vacuum or kill them. In late winter and early spring, warming temperatures may cause flies to crawl to the interior of buildings. Once indoors in walls, there is no practical way to prevent cluster flies, face flies and others from emerging. The easiest method of control is to kill them with a flyswatter or remove them with a vacuum as you see them. For large numbers of flies found indoors, use an insecticide labeled for indoor flying insects, such as pyrethrins.

Control of flies seen in spring on the building exterior is not necessary because they are trying to move away from the structure. Killing these flies has little, if any, influence on the number of flies seen inside homes. Treatment in spring also will not reduce fly populations next fall.

CAUTION: Read all label directions carefully before buying insecticides and again before applying them. Information on the label should be used as the final authority.

Reviewed 2008

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