DescriptionFungus gnats are tiny mosquito-like insects, about 1/8 inch in length. You will generally first notice them darting about new seedlings.
Being so small, they can enter your home or greenhouse through the slightest of openings. More often they come in as eggs, either in the soil of plants that have been outside for the summer or in damp bags of potting soil.
Adult fungus gnats are mostly an annoyance, but the larva can do damage to young plants and seedlings by feeding on the new, tender roots. It is also thought that they feed on the developing callus of cuttings, delaying the development of new roots.
Their feeding stresses the plants and provides an entryway for disease pathogens. The first symptom of damage is usually wilting, followed by general decline of the plant.
Fungus gnat eggs are laid in cracks on the soil surface. They hatch into larva within 6 days and begin feeding on plant roots. After feeding for about 2 weeks, they pupate in the soil and emerge in less than a week as adults, to begin the cycle all over again.
Fungus gnats are one of those insects that give birth to mostly females, helping the population to increase rapidly. One female can lay between 100-300 eggs.
Monitoring: Yellow sticky cards, placed standing up on the soil surface, attract and hold the adult fungus gnats and give you an idea of the size of the population.
Placing a slice of potato on the soil surface sometimes attracts the feeding larva. The potato slices can be used to collect and dispose of larva and to gauge when the larva are actively feeding, for timing of pesticide applications. Make sure the potato slices do not dry out.
Deterrence: If you are working in a greenhouse, do a thorough cleaning before you begin new seedlings. Soil and weeds on the floor are very attractive to fungus gnats.
In addition to feeding on plant roots, fungus gnat larva will consume organic material in the soil. Avoid potting mixes containing fresh compost, which seems to be attractive because of its high microbial activity.
Use a well draining potting mix, since fungus gnats are more attracted to soil that stays moist. Potting mix stored outdoors, where it may stay wet, will often contain fungus gnat larva.
Control: If deterrence doesn't keep your fungus gnat population under control, there are a couple of biological controls that are effective.
A form of Bacillus thuringiensis (var. israelensis), has been shown to be effective against the larva when they are at the feeding stage. It is sold under the trade name of Gnatrol. The bacteria must be eaten by the larva. Gnatrol only stays active for two days and will require repeat applications. Follow the package instructions.
There is also a type of nematode, Steinernema feltiae, that can be used to drench the soil. These tiny worm like creatures will actually enter the larva. There they release a bacterium which is lethal to the larva.
Since both controls are living organisms, you probably won't find them sitting on a shelf in the nursery. However, they are available from many catalogs, through mail order, and some nurseries will stock them during seed starting season.
Over the counter gnat or "flying insect" sprays are effective against adult fungus gnats, particularly those containing pyrethrins. Again, multiple applications may be necessary.