Winter means gardeners shift to houseplants. Houseplant gardening means fungus gnat frustration. My house is no exception.
Knowing as much as we can about the enemy gives us the upper hand; knowing the life cycle and habits of fungus gnats is key to controlling them.
Fungus gnats are small flies that infest soil and potting mixes. Their larvae primarily feed on fungi and organic matter in soil, but also chew roots. They can be a problem in your greenhouses in the summer and in your houseplants over the rest of the year.
They are about 3mm long, and have y-shaped veins on their wings (yes, you need a magnifier for this).
They don’t bite but they sure are a nuisance. The larvae can damage your plants by transmitting disease organisms like Fusarium, Pythium and Phytophthora. They also feed on roots and root hairs. This is not cool: root hairs are the part of the plant that draws water and nutrients from the soil so they’re a part of the anatomy we want to protect from damage.
The larvae are 4-6mm long and have shiny dark heads and worm-like white bodies.
Because adult fungus gnats are attracted to light, I often first notice them near the window, hanging out by my potted plants or running across the soil or leaves.
They’re sh*t fliers and tend to fly erratically at about 1-2” above the soil. If you have ones flying higher than that, and they’re much smaller than a house fly it’s probably not fungus gnats. They are often confused with shore flies and other pesky insects.
Female fungus gnats lay tiny eggs in moist soil. If conditions are especially moist, you may find yourself with an infestation of larvae that can leave slime trails on the surface of soil that look like trails from small snails or slugs.
A generation of fungus gnats—female adult to female adult—can be as fast as 17 days depending upon temperature. The warmer it is, the faster they will develop and the more generations will be produced in a year. At 24°C, they live 21 days. They will live even longer —up to 38 days— if the temperatures are closer to 16°C.
Most of that life (10 days) is spent as a larva, so that is a window for control.
So, what’s a gardener to do?
Don’t overwater. Make sure your pots can drain well and remove any standing water. Don’t water again until the surface of the soil has been dry for a couple days.
Use yellow sticky cards to trap flying adults.
Use slices of potato on the soil to attract and trap larvae. Replace with a fresh slice of raw potato every few days and throw the old piece in your outdoor compost.
Put mosquito dunk on top of the soil as it is a bacteria that kills the fungus that the larvae eat. This is a biological insecticide called Bacillus thuringiensis subspecies israelensis (Bti). It is available at most garden centers and hardware stores, so it is quite convenient for us home gardeners. Repeat the Bti applications every five days.
Fungus gnats have many overlapping generations each year so you have to keep at it for a couple months to capture all the life cycles.
Commercial greenhouse growers sometimes opt for beneficial nematodes and parasitic flies to control particularly bad outbreaks. This is a reminder to always isolate your new houseplants before introducing them into your collection. Prevention is always the most important tool.
- University of California, Integrated Pest Management
- Cornell University, Greenhouse Scout School, September 2023
- Dr Cheryl Sullivan, University of Vermont Entomology Research Laboratory
- Olds College, Greenhouse Production Course, 2021