I’m trying to figure out why we suddenly have a bunch of house flies inside. Every day for the last week we’ve killed 20 to 30 flies, mostly in the dining room. J.R., Boston, MA

For an accurate diagnosis, you really need to have the flies identified. If they’re the size and shape of house flies, they could be house flies, or cluster flies, or blow flies, among others. If they are indeed house flies, they could be coming in from a breeding source outside (animal feces, rotting vegetation, garbage, etc.), or there could be a breeding source inside (see House Flies Indoors?).

If It’s Disgusting, Flies Will Love It!

If you’re fairly confident that you don’t have anything putrid or rotting in your yard and you don’t leave unscreened windows open so flies can enter, the next thing we have to consider is where the flies could be breeding in your home. Keep in mind that flies feed on and lay eggs in anything that is rotting or putrefying. The likely breeding sites for house fly larvae indoors include animal carcasses, large numbers of dead insects, rotting meat or produce, and pet feces (see Sources for House Flies Inside a Home). I know of a case where a meat delivery for a home freezer was left as usual on the customer’s workbench in the basement. The customer forgot about it and had to deal with a rotting mess and a houseful of flies several days later!

Animal Carcasses Are a Common Source of Flies

In situations like yours though, our first thought is that there is a dead animal hidden in a wall void, attic, crawlspace, or some other out-of-the-way place. The animal could be as small as a mouse, although with that many flies emerging it’s more likely a larger animal such as a bird, rat, squirrel, raccoon, or opossum.

Flies can detect a carcass from long distances. If a female fly gets into your house, she will deposit eggs on that carcass. Eggs can hatch quickly in hot weather, the whitish maggots feed for a few days, then pupate. Adult flies emerge from the pupal case shortly after.

Often by the time we are called in, the infestation is almost over and what is left of the carcass is dried out. But, depending on the size of the animal, more than one generation of flies could develop. Your nose may guide you to the location of the decomposing animal or other breeding source.

If we can track the flies to their breeding source and if it’s accessible, we can usually remove their food source and then treat the space to kill adult flies and maggots, stopping the fly cycle. In your case, the dining room wall voids would be a prime place to start the inspection. Give Colonial Pest a call and let our technicians put their expert training to work to resolve your fly problem.

Photo Credit : By Gladson Machado – Own work, CC BY-SA 3.0, wikipedia