What are fungus gnats?

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.

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.

At Plantscape Designs Inc. ,we combat the infestation of fungus gnats with bological control such as the use of parasitic nematodes in our N.E. plantscape clients.