In the winter people who have houseplants in the home may notice tiny insects, similar to fruit flies flitting around the home. While they don’t bite humans or cause human disease, fungus gnats, (several species and families of Diptera), can be annoying. They may flit into your face or end up in your drinks, although they are not attracted to food. While they don’t harm most mature plants unless there is a very severe infestation, they can harm seedlings as the larvae feed on seedling roots. They can also spread the fungus responsible for damping off, so if you want to start seeds in your home you should get rid of the fungus gnats first.
Fungus gnats look like small dark mosquitoes. They are about 1/16 to 1/8 inch in size, with a dark, slender body shape; females have an ovipositor which can look like a “stinger” on the abdomen. Their wings are clear or pale gray, with veins visible. If you can see them closely you will notice long segmented antennae. Fruit flies are generally rounder, and lighter in body color than fungus gnats with darker wings. Fruit flies and other household flies are pretty good fliers, but fungus gnats are not as active and are often seen resting on plants. Like all flies they are attracted to light and may be seen on windows or lamps.
The larvae of fungus gnats are almost invisible to the naked eye. They live in damp potting soil or damp organic matter. They are a typical fly larva or maggot, with almost clear bodies and a black head. To see if you have them in pots, lay several slices of raw potato on the soil surface and check the side on the soil the next day. The larvae are generally attracted to the potato and will show up as small dark spots (that’s the heads) in the potato slice.
Where do fungus gnats come from?
If you move houseplants outside for the summer you may bring the gnats back inside in the fall. Many greenhouses have a problem with them so a new plant you bring home could carry the insects. Any damp organic matter you bring inside could harbor them. Planting mediums (soil) that have organic matter in them and are not pasteurized or sterilized can have the eggs or larvae of fungus gnats.
Organic type potting soils containing compost or manure and garden soil are more likely to have fungus gnats than some other soil mixes. But most potting soil mixes contain some organic matter, whether peat, coir or bark fiber. To minimize the chances of getting fungus gnats look for potting mixes that are sterilized or pasteurized. This is especially important for starting seeds. Look for bags that do not have holes and have not been left outside to absorb moisture. Don’t add your own compost or other natural products to potting mixes.
Fungus gnat Life Cycle
Female fungus gnats lay eggs in damp potting soil or other damp places which hatch in about 3 days. The tiny larvae feed on plant roots and decaying organic matter in the soil for about 10 days, there are 4 larval stages or molts of the skin, then they turn into tiny pupates or cocoons. In 4 days these hatch into adults and the cycle begins again. So depending on temperature each generation takes 17-24 days.
Fungus gnats are very hardy. They can survive freezing temperatures and live indoors or outdoors. The adults are attracted to flower nectar or pollen and can be pollinators for some plants. They can carry mushroom spores on their feet and are important distributors of spores. Their feet can also carry disease from plant to plant. In the absence of flowers adult fungus flies feed on bits of fungus on soil surfaces and may feed lightly on soft plant tissues. They don’t live long, 10-24 days.
Damage and control of fungus gnats
The worst damage from fungus gnats occurs to seedlings, which cannot tolerate much feeding on their roots. Since these insects also carry dampening off, a fungal disease that kills seedlings, it’s important to eliminate them before starting seeds. In heavy infestations the fungus gnat larvae can also limit older plants growth or in some cases even kill more delicate plants.
There are several ways home gardeners can control fungus gnats. First the gnats need very moist soil to breed in and overwatering plants can aid infestations. Let the soil surface dry between watering. Also dispose of water in pot saucers and trays soon after watering. To keep the soil surface drier you can add perlite, fine gravel like parakeet gravel, or clean sand to the soil surface. Some sources recommend diatomaceous earth but since it can be harmful to human lungs if inhaled it’s better to use safer products.
Even proper watering can’t control all fungus gnat infestations. One of the most effective treatments involves a biological product called BTi. This is not the BT product sold for use on garden caterpillars but a special product Bacillus thuringiensis subspecies israelensis for fungus gnats. It can generally be found in better garden stores or from on line nursery/garden supply sites. Brand names include Gnatrol. This is mixed with water and poured on the soil. You will need to do it every 5-7 days for a month or so. BTi is harmless to children and pets. Mosquito dunks or other mosquito products using BT are not usually effective in plant soil nor are the BT products for garden caterpillars.
Nematodes (Steinernema feltiae) which kill fungus gnat larvae can be purchased in some greenhouse supply stores. These are generally used by those with large plant collections or in greenhouse settings but if houseplant lovers can find them they are very safe and effective.
A home remedy that has pretty good success is hydrogen peroxide. Use a mixture of 1 part 3% strength hydrogen peroxide to 3 parts water and pour it in the pots until the soil is saturated. It will foam but that’s not harmful. Generally hydrogen peroxide will not harm plants but some like Peace Lily can be sensitive to it. Don’t use this on real expensive exotic plants. This will need to be repeated several times, with the soil drying out in between. If the leaf tips of the treated plant brown or the plant doesn’t look good after a treatment then don’t use peroxide again. Some gardeners have even used hydrogen peroxide full strength on plant soil with no serious effects.
Some gardeners have mixed a couple teaspoons of cinnamon with the hydrogen peroxide. The cinnamon kills the fungi in the soil that larvae feed on but not the larvae. Use this cautiously as some plants need certain fungi associations to grow well. It might be considered with seedlings, when you have a problem with dampening off and fungus gnats.
There are chemical insecticides that can also be used to control fungus gnats. These need to be systemic products that are poured on the soil. Check the label to see if the product can be used for fungus gnats and follow label directions. If you use these keep pets and children away from the treated plants. Spraying the adult fungus gnats with pesticide as they fly around or even rest on plants is usually useless and contaminates the indoor air.
You can trap adult fungus gnats either by buying yellow sticky paper traps in stores and cutting them into small pieces or making your own sticky traps. To make your own sticky traps coat small pieces of the brightest yellow paper or cardboard you can find with petroleum jelly (Vaseline). You can stick the coated paper in the tines of a plastic fork and place them in pots or use clothes pins to hold pieces on small stakes. Place some in windows and by light sources too.
There are fruit fly traps that can help to catch adult fungus gnats on the market. They may also be attracted to a bit of cider vinegar in a cup but these attractants don’t work as well on fungus gnats as they do for fruit flies.
Use the cut slices of potato on the top of soil mentioned above to monitor whether you are controlling the fungus gnats of not. Dispose of the pieces every few days outside of the house. This will also help eliminate the population of gnats. In some cases you may want to re-pot the houseplants with clean, soil less potting medium. Gently wash the soil off the roots before placing them in new soil and use a mild soap to wash the pots inside and out before reusing. Be aware that this may not eliminate all the fungus gnats and be prepared to use other control methods. Keep your potting soil sealed up and dry to avoid breeding gnats in it.
On occasion fungus gnats can breed in little used drains, in moist corners of a greenhouse with decaying bits of organic matter or in other moist places with organic matter. Drains can be cleaned with bleach and other areas cleaned up and scrubbed.
Unless you are starting seeds or growing seedling plants, fungus gnats are generally more of a nuisance than harmful. Be careful to weigh your control decisions based on that fact.
Here are some additional articles you may want to read.
The miracle of seeds
How to make houseplants interesting
What you need to know about seed starting medium
You can read the authors weekly garden blog here.