Fungus Gnats
Bradysia coprophila
Figure 1. Fungus Gnat
Larvae
Description
Fungus gnats are small (1/8 to 1/10th in.) in length, sooty gray or black, longlegged, flies. Their wings have a distinctive Y-shaped vein at the tip and their legs and antennae are long and delicate. The eggs are about 1/100 inch in length and laid in clusters. The larvae or maggots are white with black heads and are about 1/4th inch in length when full grown.
Life History
Fungus gnats are generally found associated with highly organic soils or potting media. The adult does not damage plants and normally moves around on the surface of plants and growing medium. They are poor fliers and generally considered a nuisance. Eggs are laid by the female in the soil. After 4 - 6 days, the eggs hatch and small larvae or maggots feed on small feeder roots and root hairs. They may also tunnel into the base of cuttings and feed on roots and foliage close to the soil. A flimsy cocoon is formed on or in soil where pupation occurs. After 5 - 6 days, the adults emerge completing the life cycle. Under greenhouse conditions there may be many generations per year.
Figure 2. Fungus Gnat
Adult
Damage
Damage is caused by larvae feeding on the root hairs and fine feeder roots of bedding plants. Other damage may include tunneling into the base of cuttings. Plants attacked by fungus gnat larvae will lack vigor and will have yellow leaves. Roots will have small brown scars on the surface. Both adults and larvae have been implicated in vectoring the spores of soil pathogens such as Botrytis, Fusarium, Verticillium, Pythium, and Phoma.
Nonchemical Control
Reduce the proportion of organic matter in your media, eliminate wet areas around germination chambers and floors under benches, reduce algal growth, practice good sanitation and inspect incoming plant material. Monitor adults with yellow sticky cards.
Chemical Control
Apply insecticidal soil drenches for control of larvae and foliar sprays for control of adult flies. Microbial insecticides can also provide effective controls.