Fungus Gnats

Bradysia spp. adults are 1/16 to 3/8 inch long and delicate flies with long, thin antennae and legs. They resemble mosquitoes. Females lay eggs in soil, which hatch in about 4 days. Larvae grow through four increasingly larger instars up to about 1/3 inch long. Larvae have a shiny black head and clear body, with internal organs visible through their surface. Note that moth flies and shore flies can also be problems in overly wet conditions. Learn how to distinguish the adults and larvae of these from fungus.

Young larvae chew and feed on root hairs and algae; older larvae may additionally feed on the inside of roots, on stems near growing media, and on leaves touching the soil. When populations are high, larvae may bore into larger roots or stems. One generation may be completed 21 or 40 days at 72° or 61°F, respectively.

Larvae usually feed on roots and algae within 1 inch of the soil surface. Root feeding can allow entry of plant pathogens. Direct damage can cause wilting even though the plants are being sufficiently watered and can be particularly severe in propagation areas and seedling flats. Adult fungus gnats also disseminate soil-inhabiting pathogens on their bodies and in their feces. Fungus gnat adults can be a nuisance to workers when present in large numbers.

Monitoring, biological and cultural controls, and applying certain pesticides are the main management methods for fungus gnats.

Fungus gnat larvae can be controlled by drip-irrigating, drenching, or spraying growing media with entomopathogenic nematodes (Steinernema feltiae). Soil-inhabiting predaceous mites (Hypoaspis or Stratio laelaps spp.) and a predatory rove beetle, Dalotia (=Atheta) coriaria, may occur naturally in crops and can be purchased and released according to the suppliers' directions to enhance biological control. For more information, see Biological Control, Protecting Natural Enemies and Pollinators, and Natural Enemy Releases for Biological Control of Crop Pests.

Keep production areas free of algal scum and weeds, which provide breeding sites and food for fungus gnats and shore flies. Excess humidity, overwatering, poor drainage, prolonged wet surfaces, standing water, and the use of incompletely composted and unpasteurized organic matter or manure in growing media favor fungus gnat abundance, so avoid these situations. Commercial sources of peat and almost any organic-containing growing media that becomes wet prior to use or has been stored on soil or uncovered can be infested with fungus gnats. Keep growing media covered until use and pasteurize any potentially contaminated, organic-containing media with a kiln, steam, or other heating method before use when growing crops sensitive to fungus gnat damage. Keeping flats covered with sufficiently fine-mesh screens can exclude egg-laying adults from seedlings and young plants especially susceptible to fungus gnat damage.

Bacillus thuringiensis ssp. israelensis (Gnatrol), cultural controls including exclusion and sanitation, and the conservation and release of natural enemies are organically acceptable methods of fungus gnat control.

Yellow sticky cards placed in greenhouses will capture adult fungus gnats. For more information, see Monitoring with Sticky Traps and Sticky Trap Monitoring of Insect Pests. Larval populations can be monitored with cubes or slices of potatoes pressed slightly into the soil. Inspect the underside of potato traps regularly to observe and count the fungus gnat larvae and assess changes in their relative abundance in response to your management actions. For more information on treatment decisions, see Establishing Action Thresholds.

Apply drenches to the top 1 inch of soil to kill larvae; avoid application of excessive spray that may leach or move insecticide too deeply into growing media. Pyrethrins and other adulticide aerosols, foggers, or sprays can quickly, but temporarily, reduce adult populations.

Selected Products Registered for Greenhouse or Nursery Ornamentals