Blow flies in the Home

Blow Fly. Photo: UNL Department of Entomology

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When huge numbers of flies suddenly appear inside the home, homeowners become very distraught and often don't know what to do. The flies often look somewhat like houseflies, but may be shiny green, blue, bronze or black. These shiny, metallic colored flies are called blow or bottle flies. They congregate around windows and produce a buzzing sound.

Bottle or blow flies lay eggs in decomposing organic matter, like garbage, animal manure, decaying vegetables, grass clippings and poorly managed compost piles. These flies are important in nature in the decay process of animal carcasses and are usually the first insects to arrive�within hours or even minutes�after an animal dies.

When these flies suddenly appear inside, it means one of these disgusting sources is producing flies and should be found. There are usually only a few possibilities that need to be investigated.

The first possibility is there is a break in sewer pipes where sewage is seeping beneath the pipe where flies can get to it. A few years ago, a man whose company serviced interstate rest areas called the extension office and described huge numbers of shiny bronze-colored flies that showed up in the summer. He found it strange the fly problem was only at one rest stop and only in the women's bathroom. Because of the odd location of the flies, it seemed reasonable there might be a break in one of the sewer pipes in the women's bathroom. We suggested he hire a plumber who could check the sewer pipes for cracks.

Blow Fly Maggots.
Photo: UNL Department of Entomology

Green Bottle Fly Pupae
Photo by: B. Ogg

A second possibility is garbage that has not been taken out often enough. During the summertime, flies can lay eggs in meat and vegetable scraps in your trash can and can go through an entire generation in less than a week. When fly maggots finish feeding, they often crawl away and pupate in dark, secluded places. Sometimes these maggots or pupae (shown at right) are seen by homeowners who do not know what they are. Vacuum them and discard the bag. Those missed will emerge later as flies.

A dead animal carcass will produce a flush of flies. One common source is a mouse in a forgotten mouse trap or dead inside the walls after eating mouse poison. (This is one reason why we don't recommend using rodenticides for mice inside the home.) A squirrel, bird or racoon that dies in the chimney or attic can be fodder for flies. The bigger the animal, the more flies will be produced, although it is amazing how many flies can develop in even a small animal like a mouse.

Sometimes it is impractical to find and remove the dead animal�especially when it is inside the wall. Before getting out the wrecking tools, it is helpful to know that bottle flies typically only infest "fresh" cadavers. There are other insects that show up later, such as rove beetles, carpet beetles and hide beetles, but a cadaver is only a good food source for bottle flies for a short period of time.

During the wintertime, flies are not active in Nebraska so these situations only occur during warmer months when flies are active outdoors.

Dealing with flies. These flies are attracted to the light from windows during the day so fly strips are a non-toxic method of capturing them. (They are a bit unsightly, but it's only a temporary situation.) Make sure your draperies or window treatments are securely tied back so the fabric doesn't get "stuck" in the sticky tape. Aerosol insecticides�like "bomb" treatments�can be used successfully but will only kill exposed flies. Later emerging flies won't be affected by the treatment. And then, of course, is the handy and traditional flyswatter. It takes a little more work, but many people feel a sense of satisfaction after using it successfully. We recommend the use of fly strips along with a flyswatter for this situation.

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University of Nebraska-Lincoln Extension in Lancaster County is your on-line insect pest and wildlife educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office