Fungus gnats are small flies that infest soil, container media, and other sources of organic decomposition. The larvae of most species feed on fungi and decaying organic matter and are not considered an economic problem. A few species will attack healthy root tissue of economic plants and can an occasional problem in greenhouses and vegetable transplant industry, where they can damage seedlings and plants. In addition, large numbers of flying gnats may be a nuisance to workers.
The larvae are slender with a black head and an elongated, whitish-to-clear legless body. Length when fully grown is about ¼ inch. 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 slugs.
The adults are small, delicate flies with a dark-brown body and dusky wings. They have a small head as well as rounded, moderately prominent eyes that meet above the base of the antennae. The legs and wings are comparatively long. They look more like tiny mosquitoes than common flies. When disturbed, adults run rapidly or take flight, which usually consists of short darting or hovering movements over a small area.
Survival and Spread
Females lay tiny eggs in moist organic debris or in damp organic media where larvae feed and pupate. At 75°F, eggs hatch in about three days. The larvae take roughly 10 days to develop into pupae; and about four days later, the adults emerge. A generation of fungus gnats (from female to female) can be produced in about 17 days depending upon temperature. The warmer it is, the faster they will develop and the more generations will be produced in a year.
Fungus gnats have many overlapping generations each year. Studies suggest that fungi provide an essential nutrient source for the larvae. However, if fungi are in short supply, the larvae use seedling plants as an alternate food source.
Sound management practices can help remove the conditions necessary for the development of fungus gnats and reduce the need for pesticides.
Most of the larvae are in the top inch of soil. Rogue out any old plants and rotting materials. Fungus gnat problems are exacerbated by over-wet conditions and diseased roots, and are a symptom of poor cultural practices.
Potting media should be stored dry, and pots and production areas should be well drained. Fungus gnats can exist on soil fungi, algae under benches, and on damp mossy benches. Some growers apply hydrated lime to eliminate the fungal food source.
Insecticides are rarely warranted to control fungus gnats. If high populations become intolerable, biological controls such as Bacillus thuringiensis subspecies israelensis, predaceous mites, and/or Steinernema nematodes can be used to control the larvae. Otherwise, a pyrethroid will provide temporary, fast-acting control.
An integrated pest management approach that includes good water management, practices that promote good root health, and judicious use of insecticides will keep most fungus gnat problems under control.