Have you ever noticed a small, fuzzy moth hanging out on the walls of your restaurant’s kitchen or near the bathroom? It could be a drain fly. Also called a moth fly, small fly, sink fly, sewer fly, sewer gnat, and filter fly (and less commonly a damp fly, moisture fly, pipe fly, and humidity fly), this insect makes its home in the gunk that collects in pipes. Don’t worry, though — they’re usually easy to identify and eliminate.

Drain fly

Drain flies are often confused with moths, hence the alternative name moth fly. Despite its hairy appearance and oversized wings, they are a true fly, not a moth. They’re very small, about 2 to 5mm or 1/8”, and are light gray, tan, brown, or black. You can also identify them by the vein pattern on their wings, which runs generally parallel to the fly’s body when the wings rest folded on the fly’s back.

Drain flies don’t fly well, so you’ll usually see them hanging out on walls. For the same reason, they don’t stray far from where they hatched — a drain or standing water source of some kind.

Adults don’t feed and live about two weeks.

Sewer gnats breed in damp, decomposing organic matter, like the goop that collects in kitchen and bathroom drains (and hence the name drain fly ). They do breed outside drains, though, and you can find their larvae in standing puddles or small pools, sewer lines, soil that’s been contaminated with sewage. If enough food collects on your kitchen floor or if you don’t clean your bathroom well, drain fly larvae can even live in damp or otherwise moist floors. Depending on the species, they can thrive in aquatic, semi-terrestrial, and sludge-like environments.

Drain fly larvae are tough survivors. They’ll survive high temperatures and in places with low oxygen. So, you can’t get rid of them by simply stopping the drain or flushing the drain with hot water.

Are Drain Flies Harmful?

Drain fly.

The good news about drain flies is that they don’t spread disease. Neither do they bite. And in low numbers, drain fly larvae can actually help break down material in your drains, which means you’re less likely to have clogs form.

On the negative side, drain flies can cross contaminate surfaces they land on and in that way spread germs. They also grow and multiply quickly and can thus become a nuisance. They’re even known to trigger asthma in some people.

Although they’re generally harmless, health inspectors — who often note these insects as small flies in their reports — won’t appreciate these residents in your restaurant kitchen and will require that you control them , as Indiana and other state codes indicate. Restaurant patrons likely won’t appreciate them either, especially if one ends up in their burger.

Discovering where in your restaurant the drain flies are breeding isn’t too difficult because they don’t stray far from where they breed. Simply find the drain flies and look for nearby drains.

If there are multiple drains situated near the flies, you’ll have to investigate a bit to discover exactly which drain or drains they’re breeding in. To find out where they’re growing, clean and dry off the drain inlets and then place sticky tape over the holes at night or over the weekend. Make sure to push a few holes through the tape for good airflow.

In the morning, check the tape. When adult moths crawl out of the infested pipe at night, they’ll get stuck to the tape, and you’ll be able to identify where they’re making their home. If you don’t find any moths, continue placing the tape at nights. Whether moths emerge from the pipes will depend on where in the breeding cycle they are. It may take up to 10 days for the next round of moths to emerge.

As noted, drain moths don’t only live in drains. And we’d be remiss to mention that they often breed in several sites, so if there’s more than one drain in your restaurant, you might find that several are infested with larvae. When looking for drain moths, also check floors, puddles, trash receptacles, grease, and any damp and dirty areas in your restaurant.

Credit: Bonie Bogle. “Crawl space under kitchen.” License info.

If you can’t find the sewer gnat larvae in your restaurant using the tape method, they might be breeding in a hard-to-see location. Because drain flies are so small, they can crawl up through floorboards and through cracks into your restaurant. If that’s how they’re getting in, it might mean that they’re breeding in the crawlspace or perhaps in the pipes under your building. When that’s the case, you either have too much moisture in your crawl space or one of your pipes has a crack in it, which has allowed the moth fly larvae to congregate there.

If your pipes are cracked, contact a licensed plumber to repair the problem. If moisture is the issue, consider hiring a specialist to dry out the crawl space and either encapsulate it or install a vapor barrier. Encapsulating your crawl space is a good idea for the overall health of your restaurant anyway, as it will keep away pests that would otherwise be attracted to the damp conditions under your building and it will stop fungus and mold from growing.

Other hard-to-find places where drain flies breed include sump pump basins and moist mops.

How to Get Rid of Drain Fly Larvae

Once you’ve found where the drain fly larvae are living, you can go about killing drain flies. The key to destroying the larvae really comes down to getting rid of their food source, as the larvae themselves are quite resilient. As said, they can survive high temperatures and low oxygen, so you can’t simply flush them out with hot water.

Instead, start by cleaning. Clean the outside and around the infested area. Then clean the area itself. If the flies are breeding in a drain, first pour a few gallons of water into the drain to moisten it. Then use a metal pipe brush to clean the inside. You can use a plumbing snake to pull and break up grime, as well.

Next, use a drain cleaner. Drain cleaners come in gels and foams, and whether acid-based or enzyme-based, all work to dissolve organic matter.

How to Get Rid of Flies in Your Restaurant

Killing adult drain flies in your restaurant is much easier than killing their larvae. They don’t fly well, so they can’t evade a fly swatter like house flies can. Insecticide spray is another method you can employ to kill the adult flies. Alternatively, if you have only a few flies, you can wait them out. They don’t live long, so they’ll soon disappear if you’ve addressed their breeding site.

Another way to kill the adult flies is to use dish soap. At night, set out a bowl filled with equal parts sugar, water, and white vinegar. Add a few drops of dish soap in it. Dish soap has borax in it, which kills insects. Overnight, the flies will drink the liquid and die.

Prevent Sewer Gnats

As for your drains, keep them free of clogs either by cleaning them or regularly using an enzyme gel to eat away any organic matter that might collect in them. You can also occasionally spray insect growth regulator (IGR) down the drains. Some IGRs are specially formulated for drains.

If you’re experiencing a drain fly infestation and live in central Indiana, Yes Pest Pros can help get your restaurant up to code by controlling these pests. We can identify where the infestation is located and will take the necessary steps to rid them from your building.

If your crawl space is causing issues, Yes Pest Pros can also help with that. We offer a variety of means to dry out and maintain the space via dehumidifiers, drains, sump pumps, and encapsulation. We’re also certified to treat mold and fungus in crawl spaces.

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