Fungus gnats are tiny mosquito-like insects, about one-eighth of an inch in length. Although they are not strong fliers, they can be difficult to see while in flight. You will most likely notice them resting on the potting media or darting over foliage or other surfaces.
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 outdoors for the summer or in damp bags of potting soil. It only takes only a few fungus gnats 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 might 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. The first symptom of damage to the plant 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.
Monitor and Kill Adult Fungus Gnats
To estimate the adult population of fungus gnats, use yellow sticky cards. These small, yellow-colored cards contain an adhesive on both sides. You can purchase them at most garden centers.
Place the cards upright on the soil surface. The adult fungus gnats are attracted to the color yellow. They will fly towards the cards and become trapped 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.
Prevent Infestations with Good Housekeeping Practices and Proper Soil Conditions
If you are working in a greenhouse, do a thorough cleaning before you start new seedlings. Soil and weeds scattered on a greenhouse 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 that contain fresh compost, which seems to be attractive to them because of its high microbial activity.
Fungus gnats are more attracted to moist soil, so use a well-draining potting mix and allow it to thoroughly dry out before watering again. (Don't let the soil dry out for more than a day because drought conditions might kill your seedlings.) Be extra cautious with potting mix that has been stored outdoors. It is often wet and very likely will contain fungus gnat larvae.
Control Larva with Biological Methods
Placing a slice of potato on the soil surface sometimes attracts larva that are feeding. Use the potato slices to collect and dispose of larva and to gauge when the larva are actively feeding to apply other controls at the right time. 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 is active only for two days and will require repeat applications. Follow the package instructions.
A type of nematode, Steinernema feltiae, can be used to drench the soil. These tiny worm-like creatures will enter the larvae and release a lethal bacterium into them.
Because these two controls are living organisms, you probably won't find them on a shelf in a garden center. However, they are available to order from many catalogs and some garden centers and 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. Multiple applications might be necessary.