Fungus gnats are a plant pest that can be especially troublesome for indoor plants. As the gnats move from plant to plant, they can be a disease vector for various plant diseases, causing your houseplants to wither and die. Their larvae can do damage feeding on plant roots and leaves touching the soil.
The adult fungus gnat is only an eighth of an inch long, and resembles a tiny mosquito. The adults-the vast majority of whom are females-live only a few days, spending their time eating fungus, mating, and laying eggs.
The tiny larvae feed mostly on algae, fungi and decayed plant material, but will attack live roots and leaves as well. They stay mostly at or near the surface of the soil.
Adult fungus gnats do not bite and do not carry human diseases. Fungus gnats are solely of concern due to the damage they can do to plants.
Combating Fungus Gnats
1. Limit their entry.
Fungus gnats are so small that your house would just about have to be air tight in order to not allow any openings big enough for them to get through. But that doesn’t mean it’s completely futile to keep the gnats out, or at least to lessen the number that get in.
The majority of fungus gnats that gain entry to indoors do so not by flying in or crawling in, but by being brought in when they’re still eggs. If you see gnats flying around a plant you intended to bring inside, there’s a good chance it’s infested, and is best left outside.
2. Allow your soil to dry.
Probably the single most effective action you can take against fungus gnats is to not overwater your houseplants. Fungus gnats love the moisture. Allow the soil to dry between waterings, especially the top inch or so.
The gnat eggs and larvae are less likely to survive in dry soil, and the adult females are less likely to lay their eggs in dry soil.
3. Change the soil.
You should re-pot your houseplants periodically. Older potting soil can break down and retain more moisture.
4. Clear away decayed plant material.
If there are decayed bulbs and roots and other plant material in your containers, this will attract fungus gnats and give them plenty to eat.
5. Use Gnat Stix.
Yellow sticky cards called Gnat Stix operate on the same principle as the Roach Motel: The insects get stuck in the adhesive. Place the sticks upright around your plants to catch as many of the adults as possible before they lay their eggs. You won’t get them all, but if nothing else you might get a sense of the extent of the infestation.
6. Attract the larvae with potatoes.
If you put quarter inch thick wedges of potato in your potting soil, most fungus gnat larvae will choose that over eating your plants. Give them some time to gather at the potato, then remove and discard it and them. Again, you won’t get all of them, but you may get an indication of how bad the infestation is.
7. Use insecticides.
One application of insecticide will accomplish very little. Insecticide has to be applied repeatedly to deal with each new generation.
Furthermore, insecticides that are effective on the adults will do nothing against the larvae, and vice versa. Pyrethroid-based insecticides tend to work best on the adults. The insecticide imidacloprid can be effective against the larvae.
8. Use biological controls.
Rather than trying to kill them with insecticide, you can sic the natural enemies of fungus gnat larvae on them.
One option is a form of the bacteria Bacillus thuringiensis called israelensis. This is sold by mail order under the brand name Gnatrol. Drench the soil with Bti, and if the larvae eat it, it releases a toxin that kills them. The bacteria will only stay active about two days, so you’ll likely need to use multiple applications.
Another option is the nematode Steinernema feltiae, also available by mail order. Use these only if you really, really, really hate fungus gnat larvae. These microscopic roundworms crawl into the larva’s anus and release bacteria that eat the larva from the inside out, slowly killing it over three to four days.
W.S. Cransaw and R. A. Cloyd, “Fungus Gnats as Houseplant and Indoor Pests.” Colorado State University Extension.
Marie Iannotti, “Fungus Gnats – A Pest of New Seedlings and Cuttings.” About.com.
Suzanne Wainwright-Evans, “All About Fungus Gnats.” Learn2Grow