DARKWINGED FUNGUS GNATS

James R. Baker, Extension Entomologist, Emeritus

General Information

Darkwinged fungus gnats are slender with comparatively long legs and antennae. They are grayish-black and about 1/8 inch long. The eggs are yellowish-white and tiny. Darkwinged fungus gnat maggots have shiny black head capsules and white bodies. The last body segment is lobed and helps push the insect along. Mature larvae are about 1/4 inch long. Initially white, pupae become dark shortly before the adult emerges.

Biology

Darkwinged fungus gnat maggots are important pests in some greenhouses and mushroom cellars. Besides the adult nuisance factor, heavy larval populations can damage roots and enter tender stems at soil level. They are also pests of house plants. Several of these flies are of economic concern. In greenhouses, darkwinged fungus gnats are generally most abundant in the winter and spring. Adults and larvae inhabit moist, shady areas. Adults live about 1 week, during which time each female deposits 100 to 150 eggs. They are laid in strings of 3 to 40 on soil, usually near stems of plants. They hatch within 4 days in the greenhouse. There is a tendency for the progeny of each female to be all one gender.

The larvae begin feeding on the root hairs and roots usually in the upper two inches of medium, working their way up the plant and into the stem. However, they also feed on any organic matter in the soil. Being somewhat gregarious, the larvae often form clusters in the soil. They mature in about 14 days, after which they construct a pupal case in the soil made of silk and debris. The pupal stage lasts about 3.5 days. Adults are weak fliers, but they run rapidly on the soil surface or may remain motionless to avoid predation.

In turf, they are occasionally seen in high numbers.  This is usually in high organic soils and areas of heavy thatch following wet periods.  Adjacent natural areas and mulched beds may contribute to the situation.  If a large number of eggs hatch near each other and the maggots migrate, they may form a snake-like line crawling atop each other.  These are most often noticed when moving across a sidewalk or driveway.  Though these are rarely of any consequence in the landscape, some homeowners desire control.  A pyrethroid lawn and garden spray (permethrin, bifenthrin) is adequate.  For information about fungus gnat adults indoors, see residential/urban note #29.

Control

In greenhouse or nursery settings, clean cultural practices and lack of excessive watering usually will prevent fungus gnat infestations. Repair leaky watering systems and do not allow algal buildup anywhere and not just on the benches. Since fungus gnats prefer potting mixes containing peat moss and abundant moisture, consider using bark mixes and avoid overwatering ornamental plants. Some studies have shown that fungus gnats are often introduced in the peat moss and plant plugs. Investigate your current brand.  Decoy pots of sprouting grain are attractive to females that lay eggs in these pots. Afterwards, the pots should be submerged in boiling water or the contents destroyed in some manner every 2 weeks to destroy the eggs and maggots.  Sticky card traps and fresh potato slices can be used as monitoring tools when used correctly.

Fungus gnats have few efficient natural enemies. The predaceous nematode, Steinernema bibionis, has reduced fungus gnats in mushroom houses 85 percent. Another nematode, Steinernema carpocapsae, is now on the market for fungus gnat control in greenhouses. A small, soil-dwelling predatory mite, Hypoaspis miles, feeds on fungus gnat larvae and is commercially available and compatible with nematodes and Bacillus thuringiensis var. israelensis. Some species of fungus gnats in mushroom houses have developed up to 47-fold resistance to pyrethroid insecticides.
 

Helpful links:

Kansas State Fungus Gnat IPM
UConn Bio Control Note 
UConn Pest Note
University of Illinois Note
Insects Found on Yellow Sticky Cards in the Greenhouse
U.C.Davis Note
TAMU Insect Profile

Other Resources

• Back to Insect Notes
• Horticulture Information Leaflets(HILs)
• Plant Disease Notes
• North Carolina Agricultural Chemicals Manual
• North Carolina Cooperative Extension Service

Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Web page last reviewed January, 2011 by the webperson.