How To Keep Gnats Away, According To Experts

You expect pests outdoors, but it’s exasperating when gnats flit around your face or end up in your beverage when you’re indoors. “’Gnat’ is a general term for a type of small fly,” says Matt Bertone, PhD, director of the NC State University Plant Disease and Insect Clinic. “They’re a diverse group, but people refer to a lot of the little flies as ‘gnats.’”

Gnats are tiny—and they’re everywhere—so you won’t see them sneaking indoors. But the most important thing to do is to identify what kind of gnat it is so you can use the right method for getting rid of them if it’s an ongoing problem. “You want to find the source of the issue,” says Faith Oi, PhD, entomologist and extension professor at the University of Florida. “They’re breeding somewhere.”

You’ll need to do some sleuthing to figure out where these pests may be hiding in or near your home. If you can eliminate the source of their food, you can halt their reproductive cycles and keep them away.

Ahead, here are the most common types of gnats or gnat-like pests you’ll see indoors, with tips from Bertone and Oi on how to get rid of them.

Fungus Gnats

These 1/8-inch-long delicate-looking gnats may be seen flying near the soil surface of houseplants or fluttering around the house (they’re attracted to light). Your plant may have been infected when you brought it indoors for the winter, or they can fly in from outdoors. You also may have purchased an infected plant.

These tiny flies are not harmful to people or pets, but they are annoying. The female lays her eggs on the soil surface; the ¼-inch-long larvae are difficult to see but hatch in about a week. They feed on decaying matter in the potting soil, says Bertone. Once established, the tiny flies continue to mate, lay eggs, and hatch indoors.

What To Do:

• If it’s a type of plant that can tolerate it, let houseplants dry out completely between waterings, which causes the eggs and larvae to dry out and die.
• Use sticky traps to monitor levels and show which pots have gnats; however, this won’t provide control because the traps don’t contain an attractant. They just catch any adults that happen to flutter by.
• Apply a biological control called Bacillus thuringiensis var. israelensis, or Bti for short. It is a larvicide specific to certain pests and works well to halt the reproductive cycle. Mix the granules into water and apply regularly.
• Do nothing. Mostly, fungus gnats are just a nuisance and unlikely to hurt your plants, though the more houseplants you have, the more safe havens they’ll have. They’re typically more of a concern in a nursery or greenhouse setting.

Drain Flies

These little filth flies breed in sludge and decaying matter that can build up in drain pipes, so you might see them hanging out in areas such as near kitchen drains and garbage disposals, says Oi. They resemble tiny moths and are about 1/16 inch long with yellow to brown-black hairy bodies and wings they hold over their bodies when resting.

If you’re not sure if you have them, take a clear container, smear petroleum jelly on it, and turn it upside down over the drain overnight. Drain flies will attempt to fly out and get stuck, says Oi.

What To Do:

• Use a stiff drain brush and enzymatic drain cleaner to clear out sludge.
• Flush the drain with hot water.
• If you don’t achieve control, you may have broken pipes or moisture problems that need identified. You may need to hire a plumber or pest control company.

Scuttle Flies

These flies, also known as phorid flies, are about 1/8 inch long and are known as “scuttle” because of their sporadic movements. They’re often confused with fungus gnats, but they have different behaviors. These types of flies run erratically across a surface rather than flying, and they have a hump on the thorax (the area between the head and abdomen) when viewed from the side, says Bertone.

Scuttle flies feed on moist, decaying organic matter. Breeding sources may be contaminated soil around a leaking sewage pipe, the lining of drain pipes, moist foods that have dropped behind or under appliances, potted plants, compost bins, and the inside of trash cans. They will lay their eggs in many different places, so it can be difficult to locate the source of your problem, says Bertone.

What To Do:

• Look for the food source, which is usually some sort of damp, decaying matter. Sticky traps may help you locate breeding sites.
• Check drain pans under refrigerators, air conditioners, and dehumidifiers. Even damp rags and sponges can be a source of food for these flies.
• Use microbial drain cleaners to dissolve organic buildup. But avoid the use of bleach and standard cleaners, which do not kill phorid larvae.  
• Keep compost bins and garbage cans far away from your house, and clean the containers regularly so you don’t support fly populations.
• If you can’t ID the source, you may need to hire a pest control company or plumber to check for fractured plumbing lines.

Tips To Prevent Gnats From Returning

Once you successfully identify and get rid of flying pests, don’t take a chance that they’ll return. Keep them away for good by adding these tasks to your routine:

• Keep trash cans tightly closed and take out the trash regularly to remove rotting food that may attract pests. 
• Avoid overwatering houseplants to avoid moist conditions where fungus gnats thrive.
• Clean drains and the garbage disposal to get rid of food buildup that can be breeding grounds where gnats thrive. 
• Check for plumbing leaks and repair them.
• Wipe up spills and food from floors and countertops. 
• Try candles or sprays scented with citronella, lemon, or vanilla to ward them off.