Fungus gnats are tiny mosquito-like insects, about one-eighth of an inch in length. They can be hard to see while flying, but you will probably notice them when they are darting about newly sprouted 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. It only takes a couple of fungus gnat to cause a major problem because they reproduce so quickly.
Fungus Gnat Damage
Adult fungus gnats are mostly an annoyance, but the larvae can do damage to young plants and seedlings by feeding on the new, tender roots. They may feed on the callused over-area of cuttings, delaying the development of new roots.
By feeding on the roots of young plants, undue stress is put on them as they try to establish themselves. Additionally, the damaged roots provide an entryway for disease pathogens. The first symptom of damage is usually wilting, followed by a general decline of the plant. If you notice very young seedlings collapsing or looking like they have simply rotted in place, it is probably the result of fungus gnat damage.
The Lifecycle of Fungus Gnats
To control the population of fungus gnats, it helps to know their lifecycle and when they are actively feeding. Fungus gnat eggs are laid in cracks on the soil surface. They hatch into larvae within six days and begin feeding on plant roots. After feeding for about two weeks, they pupate in the soil and emerge in less than a week as adults to begin the cycle all over again.
Fungus gnats give birth to mostly females, which enables the population to increase rapidly. One female can lay between 100 to 300 eggs.
How to Control Fungus Gnats
The best way to control fungus gnats is to employ all three of the following methods:
Monitoring and Killing Adult Gnats
Get an idea of the adult population of fungus gnats is by using yellow sticky cards. These small, yellow-colored cards have an adhesive on both sides. You can find them at most garden centers. Place the cards standing up on the soil surface. The adult fungus gnats are attracted to the color yellow. They will fly towards the cards and get trapped there by the adhesive. It is not a pretty sight, but it will give you a good idea of the size of the population while killing them in the process.
If you are working in a greenhouse, do a thorough cleaning before you begin new seedlings. Soil and weeds on the floor are attractive to fungus gnats.
In addition to feeding on plant roots, fungus gnat larvae will consume organic material in the soil. Avoid potting mixes containing fresh compost, which seems to be attractive to them because of its high microbial activity.
Fungus gnats are more attracted to soil that stays moist, so use a well-draining potting mix and allow it to thoroughly dry out before watering again. (Don't leave the soil dry for more than a day, or you could kill your seedlings with drought.) Be extra cautious with potting mix stored outdoors. It is often wet and very likely will contain fungus gnat larvae.
Biological Controls for Larva
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 the timing of pesticide applications. Make sure the potato slices do not dry out.
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 larvae. There they release a bacterium that is lethal to the larvae.
Since the last two controls mentioned 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.