Lawn gnats or fungus gnats are small, dark flies often seen hovering above the grass. The pests develop in the moist soil in indoor or outdoor plants and lawns. Fungus gnats are native insects that are a nuisance as they jump and fly as the grass is disturbed. The pests are most damaging in their larval stage of growth.


Lawn or fungus gnats are an eighth-inch long and gray-black in color. The insects have antennae and legs that are longer in relation to their bodies. The wings are dark colored. The larvae or maggots have white bodies and glossy black heads. The lobed, end section of the maggot helps it to move forward. The larvae measures a quarter-inch at maturity, and the white pupae darken prior to the emergence of the adult gnat.

Life Cycle

Fungus gnats have an average life of about a week, during which the females lay 30 to 120 eggs on the soil surface, either singly or in batches of 30. It takes four to seven days for the eggs to hatch. Under optimal temperatures, the larvae develop in eight to 20 days. The next pupal stage is three to five days, after which the mature adults emerge.


The larvae of fungus gnats infest and feed on the roots of grass and other plants, including houseplants. This results in stunted growth due to damaged roots. Fungus gnat larvae are known to cause serious damage in sod farms, nurseries and greenhouses, cites the University of California Extension. Beside damage to roots, fungus gnats in all stages of growth are likely to spread diseases by transmitting pathogens.


Because the pests thrive in moist soil, prevent fungus gnat infestation by making sure your lawn is not over-watered. Abundant decomposing grass clippings and excessive use of organic fertilizers such as manure and bloodmeal also create optimal condition for growth. Use the biological insecticide Bacillus thuringiensis var. israelensis (Bti) in lawns to kill fungus gnat larvae without harming beneficial insects or soil organisms, recommends the University of California Extension.