Fungus gnats are small, black flies which may be seen in large numbers flying around your houseplants. Get rid of them by letting the plant soil dry out.
If you have small, black, mosquito-like flies coming out of your houseplants and congregating around them, you may have fungus gnats. Fungus gnats, Bradysia, have clear wings, long legs, and antennae and are approximately 2mm in length. They scuttle around the outside of plant pots, across the soil and up and down the leaves of plants, but may also fly around the room and gather around window sills.
Fungus gnats are harmless to humans, they are not interested in you or your food and they don’t bite or spread disease. However, while the odd small black fly winging its way around your home is likely to go unnoticed, large numbers can become annoying and embarrassing. So what are the flies doing in your home and what do they want from your houseplants?
When houseplants are left outside over the summer, they may be invaded by fungus gnats which hitch a ride indoors when the plants are brought inside for the winter. Newly-purchased plants brought into your home may also be infested. More importantly, the moisture level of houseplant soil may be too high. If temperatures cool and plants are over-watered, the soil never dries out and it becomes a haven for these little flies to lay their eggs.
Fungus gnats find moist soil particularly suitable for laying their eggs. The flies live for seven to ten days, in which time the females can lay as many as 200 eggs in your plant pots. When the eggs hatch four days later, translucent, worm-like larvae with blackheads emerge and live in the soil for two to three weeks, until they pupate and become flies.
The rapidly-growing larvae feed on fungi, algae and decaying plant matter. Large numbers of larvae may also feed on your plants’ roots, causing their leaves to yellow, wilt and fade. If you look carefully in your plant pots, you may see the larvae wriggling about in the top 2 to 3 inches of soil.
To confirm if the black flies in your home are indeed fungus gnats, insert 1/4 inch slices of raw potato into the surface soil of your affected pot plants. After a few days, inspect the potato slices closely – fungus gnat larvae will be feeding on the underside of the potato.
The most important thing to do to get rid of fungus gnats is to allow the soil in your houseplants to dry out in between watering, particularly the top 1 to 2 inches of soil. Drying the soil will compromise the survival of any eggs already laid, decrease the survival rate of larvae that hatch from the eggs and discourage flies from laying more eggs in your plants. It is also advisable to re-pot your plants occasionally and remove old, decaying plant matter from plant soil.
You should notice the number of small flies in your home dwindle and disappear completely within a few weeks, but if not, pyrethroid insecticides applied to the soil’s surface will kill them. Additionally, yellow sticky cards placed near plant pots will attract the flies and trap them. A systemic houseplant insecticide containing imidacloprid will kill fungus gnat larvae.
Prevent houseplant soil from getting too wet to reduce the need for pesticides. Ensure plants are well-drained and use a plant moisture meter if necessary to determine your plants’ watering requirements.