Q: We live in a four-bedroom house built in 1995. For about a year, drain flies have been congregating behind our washing machine in the unfinished section of our basement. The finished side of the basement has a welled exit, where you go outside and climb 10 steps into the yard. At the bottom of the steps is a drain that runs under the finished section of the basement and into a sump pump in the unfinished section.

We had a plumber come out and run a camera in the kitchen sink down past the garbage disposal to check for a cracked pipe. He did not see a crack but recommended digging up the concrete in our unfinished section and replacing the pipe, because, he said, that is where homeowners always have problems. We couldn’t see digging up the basement if he did not see a crack.

We have had two pest-control companies come in; they poured chemicals down the drains on all three levels of our house, with no improvements. I also tried InVade Bio Drain and InVade Hot Spot foam, to no avail.

I have been putting glue traps behind the washing machine, and within a few weeks, the pads are loaded with dead flies. It slowed during the winter, but now that it is getting warmer, we are seeing more and more flies. Any ideas of what we can do to rid us of this problem?

A: Drain flies — gnat-size insects that congregate where there is moist, organic material nearby — don’t bite or spread diseases to humans, but they can be very annoying. They are also known as moth flies, because they have fat, fuzzy bodies that resemble moths, as seen in the photos of the sticky traps that you sent. The flies lay eggs in standing water. That can be in a P-trap under a drain, a condensation pan under a refrigerator or even a saucer under a houseplant that’s watered frequently. A broken pipe can also create an ideal environment, but that’s probably not the source. The larvae develop in the water or in a slimy layer that forms on it. The life cycle from egg to breeding adult is usually about 10 to 15 days.

Drain flies often appear when people return from vacation. Once sinks and toilets are again in daily use, they get rinsed away. But when a drain is infrequently used, as it might be in a laundry area, pouring water down the drain may offer only short-term relief, because the larvae can remain in gunk that lines the piping. The larvae can also trap air bubbles and remain submerged for a day or more, according to an entomologist at the University of Kentucky.

The key to long-term control is to clean the location where larvae develop. Sprays sold for combating drain flies, such as ExciteR 55 aerosol ($21.50 for an 18-ounce can from diypestcontrol.com), kill on contact. Although they will knock down adults flying around a drain, they won’t affect the organic buildup and thus will have no long-lasting effect.

Products designed to control drain flies by using enzymes or microbes to break down organic material often work as long as you target the place where the larvae are developing. The beauty of these products is that no plumbing needs to be done. Just pour a gel or liquid down the drain or use an aerosol that foams up and coats the pipe, said Cameron Cox, a technical support specialist for Do-It-Yourself Pest Control (800-476-3368).

Besides the ExciteR contact killer and many other pest-control products, Cox’s company also sells InVade Bio Drain ($20.25 a quart) and InVade Hot Spot ($20.93 for a 19-ounce can). But plan on re-treating the drain every other day for a week, then weekly, and don’t expect the flies to disappear for several weeks. One quart of Bio Drain is enough for eight applications on one drain.

Cleaning the drain and P-trap also works. Results will be faster, and you won’t have to purchase anything if you already have the wrenches and a brush. But this is messier and requires a little aptitude for plumbing repairs.

How does this apply to your situation? First, if the problem is just around the washer in the basement, then pouring chemicals down the drains higher in the house will probably be a waste.

Clean the washing machine standpipe and the P-trap underneath. If your washer doesn’t drain completely, as with many front-loading washers, also clean the filter area. If your washing machine drains into a tank with a pump that pushes the water up to where the rest of the house’s plumbing drains to a sewer line, clean that, too.

To clean the standpipe, use a long-handled bristle brush sold for cleaning dryer lint or refrigerator coils, such as the Holikme two-pack dryer vent cleaner kit ($8.95 on Amazon).

If a floor drain is nearby, clean that, too. You might also want to consider cleaning underneath the steps, the drain outside and the sump-pump tank. Or you can call a drain-cleaning service and ask to have your drains cleaned.

If none of this works, a construction issue, such as a broken pipe, could be the problem. But get a second opinion from a different plumber — ideally one who can snake a camera through that buried pipe to see whether it’s definitely cracked before you start digging into your basement.

Have a problem in your home? Send questions to localliving@washpost.com. Put “How To” in the subject line, tell us where you live and try to include a photo.