Gnat Infestations: Causes and Treatment

  • Written By Dan Edwards on February 2, 2021
    Last Updated: February 2, 2021

Gnats are extraordinarily tenacious and widespread. Different species can inhabit your home, garden, plants and drains.

The causes and cures for these infestations vary across family types. Knowing the reasons for their appearance — and how to remove them — will create a gnat-free environment.

The most common gnat infestations are:

  • Sewer gnats
  • Fungus gnats
  • Eye gnats
  • Biting gnats

Sewer Gnats — Psychodidae

Also known as filter flies, sink flies, moth flies or drain flies, they’re one of the easiest gnat species to identify.

You’ll often see them hovering around plug holes in sinks, showers and baths, or resting on surrounding walls, tiles and work surfaces. Slightly furry looking — hence “moth flies” — and measuring 0.11 inches to 0.17 inches, they’re harmless to humans but are unsightly.

Causes of Sewer Gnat Infestations 

Standing water and poorly maintained drains are the main culprits behind sewer gnat infestations. They drink the water and feed off fresh and rotter detritus in the drain, sink or U-bend.

As their flying ability is somewhat weak, they rarely venture from their breeding grounds. Eradicating the flies, and their eggs and larvae, should, therefore, prevent reoccurrence.

Treating Sewer Gnat Infestations

Your primary objective is to remove their food source.

Find a metal pipe-brush and clean the insides of drain grates, plug holes and upper pipes. Not only are you destroying their food source, but also where the eggs, larvae and pupae reside. Once free from grime, flush with boiling water.

If this process doesn’t completely solve the issue, research shows that the commercially-available treatment of Bacillus thuringiensis will destroy any remaining critters.

Fungus Gnats — Sciaridae

These persistent pests make their residence in and around your plants — both in the garden and the home. You can see the flying adults swarming around the uppermost parts of plants, with their eggs and larvae about half an inch deep in the soil below.

Once in the flying stage, they do little damage — apart from appearing unpleasant. The larvae, however, can munch through the stems and roots of young flora. 

Causes of Fungus Gnat Infestations

Overly moist soil promotes the growth of fungi — the primary food of the gnat larvae. The female adults, therefore, lay their eggs in this damp earth to give their younglings the best start in life.

The larvae, however, aren’t that selective and will begin to attack your plants, weakening them and causing disease. Gently turning over the topsoil will reveal this little wriggler — distinguishable by white to transparent slender bodies with a black head.

Treating Fungus Gnat Infestations

You need to destroy both the adults — before they lay eggs again — and the larvae.

Commercially-available flying insect sprays containing a pyrethroid such as cyfluthrin are particularly useful. Alternatively, eco-friendly insect killers that include oils like peppermint, tea tree and cinnamon can also remove these tiny fliers.

To eliminate the larvae, consider using the naturally occurring Bacillus thuringiensis pesticide. This treatment is safe to use around humans and animals.

Alternatively, if the infestation is over a significant expanse such as a greenhouse, use predatory mites such as Hypoaspis aculeifer, which eat the larvae.

Going forward, to prevent attracting further gnats, try not to overwater your plants.

Eye Gnats — Liohippelates

A somewhat sinister family of gnats, common to both North and South America.

They’re attracted to secretions from the eyes, nose, ears and open wounds of humans and animals. Annoyingly, they hover and land around these areas both in the home and outdoors, feeding on the discharge.

Although they don’t bite, research indicates that they can transmit conjunctivitis — pink-eye — and yaws, which is an ulcerating skin infection.

Causes of Eye Gnat Infestations

Eye gnats are prevalent in February through September. While there are no primary causes for infestations, they’re attracted to rotting organic matter where they lay their eggs and the larvae hatch and feed.

Particularly tempting to these beasties are decomposing hay, grasses, crop debris and manure. It’s, therefore, essential to reduce the amount of these materials in your garden to prevent infestations. 

Treating Eye Gnat Infestations

Due to the immense volume of these gnats across all states, area-wide control is virtually impossible.

On an individual basis, ensure your outdoor area is free from rotting vegetation. Once they’re on your land, they’re more than happy to venture inside your home and buzz around your face. Flyscreens on porches and windows can reduce the chance of a home invasion.

Also, if you want to spend some serious cash, research shows that fences that are 8 feet high or more keep eye gnats at bay, as they fly close to the ground.

Insecticides such as imidacloprid and thiamethoxam will kill them, although they’re not ideal for the environment or your health. Instead, use a DEET repellent, which has proven efficacy against eye gnats.

Biting Gnats — Ceratopogonidae

Also known as “biting midges” and “no-see-ums,” these tiny gnats can ruin your outdoor time. With jaws designed for cutting into the skin, they induce painful bites on humans and animals, which can develop into lesions.

Causes of Biting Gnat Infestations

On the plus side, biting gnats won’t take up residence in your home, although they will venture inside to search out your tasty skin.

The garden is a different matter. While not aquatic, they adore heavily moist soil, areas close to salt marshlands and ground with a high manure content. Extensive rainfall combined with warm temperatures provides the ideal conditions for biting gnat proliferation and infestation.

Treating Biting Gnat Infestations

Prevention is always better than cure. Where possible, try to ensure that outdoor areas are well-drained and free from any standing water. Adding sand to soil areas may improve drainage and deter these unwanted critters.

Research, published in the Journal of Vector Ecology, illustrates that carbon dioxide traps are particularly useful and can be purchased for the home.

Again, fly screens can prevent biting gnats from entering your house, but be selective. They can pass through 16-mesh grade screens, so you’ll require a size smaller than this.


Gnat infestations can ruin your indoor and outdoor time — don’t allow this to happen. Identify the gnat species by their behavior described above and take the appropriate prevention and treatment steps. Following this advice will provide you with a more pleasant and gnat-free life.