About Fungus Gnats

Fungus gnats are tiny insects that are frequently found in greenhouses and around indoor potted plants. They are attracted by moist conditions and lay hundreds of eggs in warm, damp soil. The eggs hatch quickly and tiny larvae burrow into the soil, where they feed on several types of fungus and sometimes the roots of plants. Although they generally do not kill plants, they are a nuisance and may interfere with growth, flowering and fruiting.

Identification

Fungus gnats are tiny insects that bear a certain resemblance to mosquitoes, although they are much smaller. They may be grey or black in coloration and have delicate translucent wings. Like a mosquito, they have an elongated body and thin, spindly legs. Adult fungus gnats are poor fliers; they are usually seen walking on the soil surrounding plants.

Effects

Although fungus gnats are often only thought of as a nuisance, they can cause serious damage to plants. Adults are harmless. However, the larvae burrow into the soil or media plants are grown in and feed on the roots. This may lead to stunting of plants and crop loss in commercial greenhouses. It is suspected, although not certain, that fungus gnats may contribute to the spread of plant diseases. Fungus gnats are harmless to people and animals; they do not bite or carry harmful microbes.

Prevention/Solution

Several options are available for controlling or eliminating fungus gnat infestations. The most popular is the use of Bacillus thuringiensis var. israelensis, a type of bacteria that kills fungus gnats without harming the affected plants. This product is often referred to as B.T., and is sold under the brand name "Gnatrol." Other control measures used with high levels of success include sticky traps, parasitic nematodes such as Scanmask, and pyrethrin, which is found in many brands of chemical insecticide.

Considerations

The presence of fungus gnats may indicate a problem in the management of potted plants or a greenhouse. Most commonly, this is a result of over-watering, but certain types of planting media may also lead to favorable conditions for mold growth and fungus gnat infestations. Compost that has been aged for less than one year decays rapidly. This creates problems such as mold growth, poor aeration of soil and attracts fungus gnats.

Expert Insight

Raw potatoes can be used to detect fungus gnat larvae and to monitor response to control measures. The larvae are attracted to cut potatoes and will gather to feed on its surface. Potatoes are usually cut 1/4 to 1/2-inch thick for this purpose, and slices are placed on the surface of the soil or potting media. After four hours, the larvae on and under the potato slices may be counted to determine how severely infested a plant is. Repeating this test following control measures can give valuable insight into whether the method chosen was successful.