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Facts, Identification & Control


Often mistaken for mosquitoes, gnats are “true flies” and belong to the order Diptera. The relatively small insects are usually poor fliers and infest residences as well as commercial and industrial sites. There are over 600 species of gnats found in North America. The most common species found inside and around buildings are fungus gnats and dark-winged fungus gnats. They can be a nuisance or cause significant damage to ornamental plants and garden crops, while some species such as the buffalo or turkey gnat, also known as the black fly, attack humans and other animals to feed on blood.

Appearance / Identification

What Do Gnats Look Like?

picture of midge gnat
Most gnats are smaller, with less robust bodies, than other species of flies. Ranging in length from 1 to 13 mm, gnats are delicate-looking insects with long wings and spindly legs. Like all insects, the body is divided into three distinct parts: head, thorax and abdomen. The head has a pair of antennae that exceed the length of the head. The pests generally appear black, grey, brown, or even yellow in colour. Fungus gnats have distinct elongated coxae, structures that connect the legs to the thorax.


Gnats live throughout Canada and the rest of the world. Found in greenhouses, nurseries, and sod farms, certain species of gnats stay close to overwatered and fungi-ridden plants to mate and feed. Other species are found almost exclusively near running water, especially fast-moving rivers and streams.


The dietary habits of gnats vary by species and even by gender. Female black flies, for example, are blood feeders that take blood meals from humans, domesticated pets, and livestock, while males primarily feed on nectar. Fungus gnats, on the other hand, feed on a variety of plants, but the larvae feed mainly on the fungi that grow among overwatered vegetation or in decaying organic matter.

Life Cycle / Reproduction

Like many insects, gnats hatch from eggs as larvae and develop into pupae before emerging as mature adults. Female gnats can lay up to 500 eggs in a lifetime and depend on the presence of moisture to help their offspring mature. Black flies, for example, lay their eggs in running rivers and streams by attaching them in clusters to vegetation and rocks. The emerging larvae likewise attach themselves to plants and rocks by using tiny hooks on their abdomen and pass through six instars while filter-feeding on vegetation in the water. Pupation occurs in silk cocoons underwater, with adults emerging in air bubbles as fully mature gnats.

In contrast, fungus gnat eggs and larvae mature in moist soil, where most of the damage to host plants occurs as the developing larvae feed on organic mulch, leaf mould, grass clippings, compost, root hairs, and fungi. Many types of gnats can produce multiple generations in a single year.

Problems Caused by Gnats

Generally a nuisance pest, adult gnats become active and swarm during the warmer months. Certain species can directly affect the growth of potted, ornamental, and greenhouse plants. When plants become overwatered and the soil is not allowed to dry properly, larvae may attack the fungus that ends up growing, resulting in damage and stunting the growth of the plant.

Other species, such as the buffalo gnat, may actively attack humans, pets, and livestock to feed on their blood. Though most North American species of gnats are not known carriers of diseases, some tropical species of black flies transmit parasitic nematodes, which cause onchocerciasis, or river blindness, in humans. Additionally, large numbers of black flies attacking an animal all at once can result in death by blood loss or anaphylactic shock.

Detection / Signs of Infestation

The surest sign of infestation is the sight of adult gnats flying around. In particular, the presence of large populations of swarming gnats usually indicates an incursion problem. Noticing damaged plants near ground level and seeing larvae in the soil also clearly signifies a gnat infestation.

Prevention Tips

Exclusion represents the most effective form of prevention. Avoid overwatering plants to reduce fungus gnat populations and breeding grounds, and get rid of currently infested soil and vegetation. Application of appropriately labelled insecticides in affected areas may help prevent infestations of black flies or other gnat species that commonly travel great distances to locate food sources.

Control / Removal

Most gnats are attracted to light, the use of light traps can be effective in controlling gnat populations. For black flies, wearing light colours and long sleeves will help keep the pests at bay. Since gnats are so persistent, controlling the insects at the larval stage of the life cycle proves to be the most effective. Finding and eliminating breeding conditions and sites is the most effective long-term control method. It may be necessary to contact a trained pest management professional for the complete control and management of gnat infestations.

Fungus Gnats