Why Do So Many Flies Come Into Your House During the Summer?

House flies and blow flies share similar life cycles, with both reproducing only in the warmer months. They both look for disgusting places to lay their eggs; house flies prefer animal feces or garbage, while blow flies lean toward the bodies of dead animals. In seven to 14 days, these eggs can complete the life cycle through larva and pupa to adult. When the eggs were laid in or near your house, they often fly around inside to seek food and a place to lay more eggs. The most likely place for house flies to lay eggs inside your home is in your garbage can. Blow flies likely come from animals such as squirrels and mice that die inside your walls, under your crawl space or near your home's foundation, although they also breed in animal feces.

When the flies come through cracks in your walls or through open doors and windows it's often difficult to get out. They fly toward the light of windows, but when the windows are closed, the flies can't escape. Blow flies won't breed additional generations once inside your home, unless more animals die inside the walls or in the attic, for example. House flies, however, can lay eggs inside your garbage can before you even know you have a problem. They gravitate toward sugary liquids for food, but they'll also land on anything sitting out, including meat, which blow flies also enjoy. Because both species thrive in bacteria-ridden areas, they carry that bacteria with them when they touch your food. This can transmit diseases to you when you eat the infected food, including salmonella, E. coli and cholera.

Many insecticides aren't effective against flies because they breed so quickly; the eggs hatch shortly after you kill the existing adults. Instead, try sticky fly strips to catch the adults. Placing them near windows or light sources works best. Fly swatters also are effective, but disinfect the area where you smashed the fly to ensure no nasty bacteria is left behind. Screening all windows and closing exterior doors quickly can help keep flies out. The flies will disappear when cold weather hits, but house flies might leave eggs to overwinter and invade your house next summer.

Just when you think you'll get relief from summer flies as the weather cools, you might notice flies that hang around in the winter -- sometimes in large numbers. Although these flies are a nuisance, they don't carry bacteria like house and blow flies. They live in a semi-dormant state as adults in the winter, typically in the upper areas of your home such as a second story or attic. They fly around occasionally, but usually, you'll notice them congregating in warm areas. They don't eat indoors in the winter, and they lay eggs in earthworms, so they won't breed inside your home. To rid yourself of the flies, try an insecticide bomb containing permethrin.