Once considered only a nuisance pest of greenhouses and homes, the fungus gnat causes direct damage to plants and can be a serious greenhouse pest. The most visible sign of fungus gnats is the tiny (1/8-inch long) black adult that flies just above the soil surface. It is the larvae, however, that feed on plant roots and even on the new callus of cuttings. In serious infestations, root damage is severe enough to cause the plants to wilt.
Fungus gnats thrive in moist, wet environments ― especially where there is decaying plant matter. In short, greenhouses are an ideal place for them. An adult fungus gnat has wings but isn’t a particularly strong flier. Its long legs, however, allow it to run quickly across the soil surface. An adult female may lay as many as 100 eggs in her short, one-week lifespan. The eggs stay on the soil surface for four to six days until they hatch. Eggs are very difficult to see, since they are rather transparent and very small. The larvae that emerge are legless and either white or clear with a black head. They are about ¼-inch long when full grown. Larvae begin feeding, and depending upon environmental conditions, they may continue feeding for 10 to 140 days before forming pupae. Just three to four days after the pupae forms, the adult gnat emerges.
Larval feeding damage isn’t limited only to the physical harm to the roots. Wounds from feeding are open to pathogens such as Pythium, Verticillium, Thelaviopsis, Phytophthora and Fusarium, and studies have shown that adult gnats can transport spores of these diseases, as well.
Monitoring For Fungus Gnats
It is usually not difficult to detect fungus gnats in the greenhouse. Besides the flying adults, which can be seen with the naked eye and trapped with yellow sticky cards, larvae can be found just under the soil surface. It is suggested to put a piece of cut potato on the soil, with the cut surface against the soil. After 48 hours, pick up the potato and look for the larvae with a hand lens. Larvae also produce thin webs that can stretch across the soil surface.
Fungus gnats are most prevalent in moist, humid areas, so paying attention to water and humidity levels is key to controlling them. Keep potting mix dry until it is used, and if bagged, try to minimize tears or openings that allow the gnats to enter. Check the soil of incoming plants or plugs for evidence of fungus gnat infestation. Greenhouse sanitation is important ― keep floors and under-bench areas free of soil and weeds that can play host to fungus gnats.
Biological controls include Bacillus thuringiensis var. israelensis (Gnatrol), a bacteria that infects the larva. A predatory mite, Hyoaspis miles, feeds on larvae, as does a beneficial nematode, Steinernema feltiae. Insect growth regulators can be effective against larvae, and there are a number of traditional chemicals for control of both larvae and adults. Because life cycles overlap, multiple applications are needed.
Antonelli, A.L., and D.F. Mayer. Fungus Gnats. Washington State University Extension. http://cru.cahe.wsu.edu/cepublications/eb1573e/eb1573e.pdf
Osborne, Lance S., and W. Chris Fooshee. Fungus Gnats. University of Florida, Mid-Florida Research and Education Center. http://www.mrec.ifas.ufl.edu/lso/entomol/fungnat/fungnat.htm
Pundt, Leanne. 2006. Fungus Gnats are Serious Pests. University of Connecticut Integrated Pest Management. http://www.hort.uconn.edu/ipm/greenhs/htms/fngnatser.htm