This simple expert trick actually eliminates drain flies for good

There’s nothing worse than sighting a drain fly. You could be washing your hands or brushing your teeth, just minding your own business, until, all of a sudden, a tiny winged insect crawls out of the abyss, unannounced. If you’ve recently spotted a bug with a grayish-black hue and fuzzy, oversized wings climbing out of your drain, you’re probably dealing with an infestation. While these little critters don’t sting, bite, or cause any foundational problems, they carry bacteria and can cause respiratory irritation. Plus, if you notice one, there are almost always hundreds more lurking in your pipes. “A single female can lay up to 300 eggs in 48 hours, the larvae hatch within about two days, and they mature into adults in as little as eight,” says Mike Usry, president of Southland Organics.

What’s more, drain flies are a symptom to an underlying plumbing issue. “You don't have a fly problem, you have a dirty pipe, and until you clean the pipe, the flies are just going to keep hatching,” Usry says. Like most household insects, drain flies invade living spaces because they’re on the hunt for a food source. Since they feed on biofilm, a slimy layer of soap scum, hair, grease, or food residue that coats your drain pipes, if your plumbing is backed up or poorly maintained, you’re inviting these pests inside. Drain flies flock to this buildup (often found in sinks in the kitchen, bathroom, and guest room, plus drains in the shower, basement, or garage) before eating through it and laying their eggs.

Getting rid of them requires tackling the drain, not the bugs themselves. “Killing the adult flies accomplishes almost nothing,” Usry explains. “Sprays, fly traps, boiling water, and the classic baking-soda-and-vinegar trick all target the flies you can see, but they don't remove the biofilm that the next generation is already hatching in.” To kill the masses, you’ll need to banish the food source. Ready to end your infestation? Here’s how to eliminate drain flies once and for all.

Step 1: Gather Your Materials

To effectively eliminate drain flies, you’ll need to assemble the proper tools for the task: a stiff drain brush, clear packing tape, and a biological or enzyme-based drain treatment—that’s it. “You don't need a sprayer, a fogger, or an exterminator for a residential drain fly problem—and if someone tells you that you do, they're selling you the wrong thing,” Usry advises.

Step 2: Dry Each Drain

Because drain flies thrive in moist buildup, drying out your drains will make their current home less hospitable. Avoid running water through your drains overnight—or, if that isn’t possible, hold off on using the faucet for at least several hours. To sop up extra moisture, wipe the drain opening and the basin around it with a clean microfiber cloth.

Step 3: Apply Clear Tape

Next, Usry recommends identifying the source of the infestation, so you can accurately target the problem area. Simply cover each drain opening with a piece of clear packing tape, applied sticky-side down, while leaving a small gap for airflow. To determine whether the targeted drain is a breeding ground, keep the tape on overnight “In the morning, the drain producing the flies will have them stuck to the tape,” Usry says.

Step 4: Scrub the Pipe Walls

After locating the infested drain, scrub its pipe walls with a stiff drain brush to break up the top layer of film. “Work it as far down the pipe as it will reach, scraping the sides—not just the opening,” Usry instructs.

Step 5: Apply a Drain Treatment

Since the drain brush won’t be able to tackle the entire pipe, you’ll need to follow up the manual labor with a store-bought solution. “A biological surfactant or an enzyme drain treatment breaks down the remaining grease and organic matter chemically throughout the run of the pipe, so there's nothing left for larvae to eat,” Usry says. Following the manufacturer’s directions, pour the recommended dosage down the pipe, and let it sit overnight.

Step 6: Repeat the Process

For lasting results, Usry suggests repeating the process once a week for three to four weeks. “Because eggs are already in the pipe and the life cycle runs eight to 24 days, a single treatment kills the current generation and misses the next one,” Usry points out.

Step 7: Prevent a Future Buildup

After continuing the cycle for several weeks, prevention is the best way to keep drain flies out for good. Usry advises flushing out grease and food particles from the kitchen drain, running unused faucets (like in the guest bathroom) every week, and applying a monthly treatment if needed. By following these simple steps, you’ll keep your pipes fresh and free of drain flies for the foreseeable future.