Fungus gnats are tiny flies that don’t bite or spread human pathogens/diseases. The only way they can harm us is through frustration as they invade our houseplants or spring seedlings.

Adult Fungus Gnat

This fungus gnat adult is caught on a sticky card.

Photo Credit: ©2002 Buglady Consulting

Fungus Gnat Larvae

Fungus gnat larvae feed on highly organic soils and can damage plant roots.

Photo Credit: ©2006 Buglady Consulting

What do fungus gnats look like?

They are small, grayish to black flies that are 2 ½ millimeters long and resemble tiny mosquitoes (minus the bloodsucking). Their legs are long and slender, and their skinny antennae are usually longer than their heads. Their wings are shades of gray. Fungus gnats are relatively weak fliers and generally remain near potted plants, often running or resting on soil or leaves.

What kind of damage do fungus gnats do?

They may not threaten human health, but with fungus gnats you get a triple whammy: Not only do people hate to see them in general, the pests can be vectors for plant diseases – not good! “What’s a vector,” you ask? It’s an organism that transmits a pathogen, so if you have a sick plant, fungus gnats can spread it to all your healthy neighboring plant friends. They can also vector several different fungal root rots, including ones called Fusarium and Pythium, and even foliage pathogens like Botrytis. And as if that’s not bad enough, fungus gnat larvae make breakfast, lunch and dinner out of your plant roots.

So there’s good news, and there’s bad news: The good news is adult fungus gnats only live about one week. The bad news is that in this short time, the female will deposit 100-150 eggs on your plant’s soil surface. These eggs are laid in strings of three to 40 and can hatch within four days of being laid!

The emerging larvae are clear to creamy-white and can grow to about 5 ½ millimeters long. They have shiny black head capsules. The larvae feed on tasty root hairs in the upper 1 centimeter of the soil, then work their way up into the plant stem. (They also love to feed on the roots of your newly planted seed, so watch those seed-starting trays in spring!) The larvae feed on highly organic soils, too. After feeding for approximately 14 days, the larva pupates. In about three and a half days, an adult will emerge from the case. The total life cycle takes two to four weeks.

How do you control fungus gnats?

They key is prevention, and you can do this two ways: The first is to avoid overwatering your plants. Overwatering, to fungus gnats, is like laying a big steak on the floor in front of a starving dog – they can’t resist it. The second way to prevent the problem is to inspect the soil of a plant before bringing one home. Do you see gnats buzzing around it? If so, that’s not a good sign. Put the plant down and just walk away.

Some good monitoring methods can help cut down fungus gnat issues, too. Yellow stick cards (small, yellow cards with sticky adhesive on both sides) often do the trick. These can be purchased online. Many insects, including flies, are attracted to the color yellow. So upon seeing the yellow stick card, they’ll mindlessly fly right into it, and SPLAT! The adult fly is stuck. Ta-da! These cards are most effective when placed horizontally near the surface of potting soil. You can use popsicle sticks or straws to hold the cards, or some come with sticks in the package. Keep in mind, however, that this control method only traps adults – not the larval stage of fungus gnats.

Another way to control these plant invaders is with the potato trap method: Cut chunks of potato into 1 ½-inch-square pieces. Place them on the surface of your potting soil. This is like a chuck wagon call for fungus gnat larvae! They’ll head straight for it and start munching. Leave the potato for a few days and then lift it up – you’ll quickly discover if you have larvae in your soil. (And obviously remove the potato once you’re finished with it – no one wants rotting potatoes in their pots … or anywhere in the house, for that matter.)

Controlling fungus gnats through biological means is a relativity easy thing to do as well. Currently two different biological control agents are available: beneficial nematodes and predatory mites.

There are several species of beneficial nematodes that can be found through the Internet. These nematodes are so small, they can’t be seen with the naked eye. (If you get real close and look hard enough, you can see them with a 10X hand lens.) Steinernema feltiae is the most commonly used nematode species for fly larvae control. Nematodes are simple to apply – just water them into the soil.

One predatory mite you might like to try is Hypoaspis miles – a brownish soil-dwelling mite less than 1 millimeter in size (now that’s really small). This predatory mite inhabits the top layer (at a depth of 1-4 centimeters) of the soil and feeds on harmful soil insects, such as fungus gnat larvae and thrips pupae. It takes an average of 18 days for this mite to reach adulthood. Adult mites can kill up to seven fly larva per day. One of the favorable characteristics of this particular mite is its high tolerance to starvation. Newly emerged adults can survive for three to four weeks without food. (If fed, adults can be active for four to five months.) These, too, can be purchased through the Internet.

A fungus gnat outbreak can cause quite a headache, but by following proper water-management practices and treating the pests once they’re found in the soil, they can be easily controlled.