Fungus Gnats

by Dr. Carlos Bogran, Assistant Professor, Department of Entomology and
Scott Ludwig, Extension Program Specialist in IPM, Texas A&M University

Figure 1: Typical fungus gnat

While fungus gnats are not generally viewed as key pests, their immature stages can cause severe damage to plant cuttings and seedlings. Fungus gnats are small flies (Sciaridae) that resemble mosquitoes (Figure 1). Adult fungus gnats are gray to black, 1/8-inch long, slender and very delicate. They have long legs and one pair of clear wings. Adult fungus gnats are generally seen flying close to the potting media. Immature gnats are white legless larvae with a shiny black head capsule and may reach ¼ inch in length before pupation (Figure 2). Pupae occur in silk-like cocoons in the soil.

Fungus gnats lay their eggs on the soil surface or on plant tissue in contact with the soil. However, they generally lay more eggs on decomposing plant tissue than on healthy tissue. Females may lay up to 100 to 300 eggs in their 7-10 day lifetime. Larvae hatch from eggs 3 to 5 days after oviposition. Several generations may overlap during a growing season. Larvae feed on organic matter in the potting medium but may also feed on small plant roots and root hairs. The larvae are capable of burrowing into the plant stem at or bellow the soil line. Fungus gnats are especially damaging to seedlings where they can cause the destruction of the entire root system. Direct damage to roots may cause wilting even when plants are being properly watered. In addition to their feeding damage, fungus gnats are capable of introducing and spreading plant pathogens that cause damping off, Verticillium and Fusarium wilt, and root and stem rots. Large populations of fungus gnat adults can also be a nuisance.

Managing fungus gnats starts with good sanitation practices and prevention tactics.

• It is important to remove plant debris, old potting media, and any diseased plants from growing areas. These may become the source of future problems.

• Avoid using potting media containing organic matter that has not been completely composted as it creates ideal conditions for fungus gnat development and population buildup.

• Keep growing areas well drained and avoid overwatering; moist conditions are required for fungus gnat survival.

• Eliminating algae from underneath benches or other surfaces may also help reduce populations.

Fungus gnat damage may be prevented with timely detection and regular scouting efforts. A good monitoring program will help detect fungus gnat populations when they are at low levels, and will help in assessing control effectiveness. Adults may be monitored using yellow sticky cards near the surface of the potting medium. Sticky cards need to be checked and replaced weekly. Larvae may be monitored using potato disks (1 inch diameter, ½ inch thick). Press a potato disk into the medium and check for larvae on the underside of the disk the next day.

Figure 2: Gnat larvae

There are several biological and chemical control options available to manage fungus gnats. Commercially available Steinernema nematodes, the predatory mite Hyposaspis miles and the biological insecticide Bacillus thuringiensis subspecies israelensis provide effective control of larvae. Insecticide applications may be warranted under heavy fungus gnat infestations and usually involve surface sprays against adults and drench applications to the media against larvae. Always read and follow directions on product label.

Note: (This material appeared in the web periodical Horticulture Update, Drs. William C. Welch and Douglas F. Welsh, Editors, Department of Extension Horticulture, Texas A&M University, College Station, Texas)

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