How can we avoid dead flies in the cottage in spring?

Dead fly

Each spring when we open up the cottage, we find thousands of tiny dead flies inside. They look somewhat like fruit flies. Is there something we can do to avoid this?
—Sue Neill, via e-mail

Without seeing a specimen or a photo, or at least having more information about the situation—knowing when the insects came inside, what they’re attracted to, or how they behave—it’s impossible to know exactly what kind of bugs you’re dealing with.

According to Doug Currie, the curator of entomology at the Royal Ontario Museum, “the only flies that habitually come into the cottage in the fall and overwinter are cluster flies.” However, cluster flies are large—on an insect scale, at least—and they’re often mistaken for houseflies.

If your guests are tiny and look like fruit flies, it’s possible they are fruit flies. Other candidates include phorid flies (they’re small and humpbacked, and resemble fruit flies) and midges (they look like mosquitoes, but with shorter wings and no visible proboscis).  Or they may not be flies at all. “A lot of things can find their way into your home,” says Currie. “Insects are very resourceful.”

However, Bob Anderson, an entomologist with the Canadian Museum of Nature in Ottawa, suspects your intruders are fuzzy, grey-brown moth flies (also called marl or drain flies), whose larvae feed on the “debris and gunk” in household drains. The layer of gelatinous organic scum that accumulates in a grimy kitchen sink drain makes an ideal egg-laying spot for the females. The larvae pupate, emerge into the kitchen, go through their life cycles, and hang out in your cottage until they die. (Worst guests ever.)

Before you close up in the fall, make sure the drains are clean (scrub down as far as you can with a stiff, long-handled brush). Anderson suggests you pour a 1/2-cup of bleach down to kill any larvae and pupae. (If you’re on a septic system, be sure to use bleach sparingly, as it can destroy important bacteria in the tank.)

And, of course, follow all the usual pest-prevention measures recommended for the cottage: Seal up cracks and crevices; get rid of moisture sources inside and standing water outside; make sure there is no organic material or compost left in the kitchen; and tightly seal up all food, including dry goods such as dog food. After all, as Currie points out, “if it’s not fodder for flies, it’s fodder for something else.”