Fungus gnats

Overview

Fungus gnats, insects that belong to the fly family Diptera, occur around damp, decaying vegetation, algae, and fungi. These tiny gnats can appear in large numbers in or around buildings, prompting complaints, and also can be a problem in greenhouses, nurseries, and interior plantscapes. Fungus gnats infest soil and container media, where larvae feed on organic matter and roots, feeder roots, and root hairs.

Symptoms and Diagnosis

Adult fungus gnats are primarily a nuisance. They can enter buildings as flying adults and develop indoors through all life stages. They do not bite people or animals and, in the United States, are not known to carry human pathogens. Adults feed very little, consuming only liquids, such as water or flower nectar.

Fungus gnat larvae do damage plants. When their preferred food choices run out, they feed on roots, stunting plant growth, and causing foliage to yellow and leaves to drop. Larval damage can be especially serious in greenhouses, nurseries, and sod farms where they harm seedlings, cuttings, and young plants without fully developed root systems. Both larvae and adults can spread plant pathogens and may promote disease in commercial crops. They have been implicated in the transmission of plant fungal diseases, including black root rot, Pythium blight, Verticillium wilt, Botrytis blight, and Fusarium wilt.

Outdoors, little serious damage is done by fungus gnat larvae. Their population is kept in check by natural predators, weather, and seasonal changes. Any root feeding in gardens or landscapes is usually minor in comparison with the gnats’ beneficial role as decomposers converting dead vegetation into nutrients for plant growth, as important pollinators, and as food for small animals such as birds, reptiles, and beneficial insect predators.

Fungus gnats are sometimes confused with other small flies not discussed here, including black flies, midges, mosquitoes, shore flies, moth flies, and March flies. Adult fungus gnats are dark, delicate-looking insects, similar in appearance to mosquitoes. They have slender legs and segmented antennae that are longer than their heads. Adults commonly are about 1/8 to 1/16-inch long with wings that are light gray to clear. The common species have a Y-shaped wing vein. Fungus gnats are relatively weak fliers and remain near plants running or resting on growing media, foliage, or plant litter. They have mandibles for gnawing and tunneling.

Females lay tiny eggs in moist organic debris or potting soil. The ¼ inch-long larvae have a shiny black head and an elongated, whitish-to-clear, legless body. They eat organic mulch, leaf mold, grass clippings, compost, root hairs, algae, and fungi. If conditions are especially moist and fungus gnats are abundant, larvae can leave slime trails on the surface of media that look like trails from small snails or slugs.

Life Cycle

Fungus gnats develop through four stages: egg, larvae (four larval stages or instars), pupa, and adult. They produce many generations in a year. Adult females deposit 30 to 200 whitish-yellow eggs singly or in clusters in crevices or cracks on the surface of growing media, and in moist, organic debris. They prefer to lay eggs where the fungus is growing. Indoors, they occur any time of year.

The larvae feed for about 2 weeks and then pupate near the soil surface within thread chambers. After 3 to 7 days in the pupal stage, adults emerge and live for about 8 days. The gnats develop from egg to adult in 3 to 4 weeks. Their life cycle is dependent on temperature. The developmental time increases as temperature decreases.

Integrated Pest Management Strategies

1. Live with the gnats. If damage is minimal, the prudent thing to do is do nothing.

2. Use physical and cultural controls to manage the gnats. Screening windows/ doors and reducing moisture are recommended. Maintaining good sanitation is vital. Dead plant material and debris must be picked up. Spilled growing media and algae must be cleaned. Overwatering and sloppy irrigation need to be avoided. To kill larvae, allow the soil to dry as much as possible (without damaging plants) between waterings. Provide good ventilation. Dry, level, weed-free, well-drained greenhouses floors eliminate breeding larvae.

3. Avoid problems. Do not bring infested plant containers indoors.

4. Traps. Use yellow sticky traps for trapping adult flies and monitoring their population.

5. Biological controls. Use biological control agents such as nematodes, rove beetles, and mites to control fungus gnats. Bacillus thuringiensis subspecies israelensis is effective against larvae in potted plants. It is toxic for 2 days and doesn’t kill egg-laying adults, so repeat applications are necessary. It is best used as prevention. Steinernema feltiae is an insect-killing nematode that can be applied as a drench treatment. The best results come from a first application at planting and then following with 2 to 3 weekly applications. Ineffective when used to reduce a serious infestation. 

6. Use chemical measures. Sprays containing pyrethrins control adult fungus gnats. Because new adults emerge daily, repeat sprays every few days for at least two weeks to reduce populations. Other chemical controls include bifenthrin and cyfluthrin.

Organic Strategies

Strategies 1, 2, 3, and 4 are strictly organic approaches. For an organic approach to Strategies 5 and 6, consult the Organic Materials Review Institute (OMRI™) for appropriate Bacillus thuringiensis subspecies israelensis (Gnatrol and Summit Mosquito Bits) and pyrethrin products.

Pesticide Disclaimer: 

Always follow the product's label and ensure the product is effective against fungus gnats. Not following the pesticide label before usage is a violation of federal law.