Fungus gnats can be a problem with houseplants or plants that were outdoors but brought inside for the winter. These insects are obnoxious, small flies that infest soil or potting mix.

You might first notice them flying near windows since they’re attracted to light. However, fungus gnats will most often remain near potted plants. Adult fungus gnats don’t damage plants nor are they a health hazard to humans. However, fungus gnat larvae in the soil can cause plant wilting, poor growth or yellowing. These insects are notorious hitchhikers; you can bring them indoors with any container plant that spent time outside. They thrive in moist soil.

Identification

These insects are a nuisance! If you see very tiny gray or black fruit-fly like flies about 1/8-inch-long hovering around your houseplants, chances are you have fungus gnats. You can capture adults by placing yellow sticky cards in your plants.

The really bad news is that adult flies can lay up to 200 eggs during their short lifetime in potted plant soil. Within four to six days, larvae will hatch. You can check for larvae by placing a slice of potato down into the soil, enough to cover the cut edge of the potato. If larvae are present, you’ll find them feeding on the potato in a few days.

What to do

You need to understand how you get these insects in the first place in order to control them. Damp soil will allow fungus gnats to thrive; so, avoid over-watering. If you bring outdoor plants inside, consider repotting them in fresh, quality potting mix. When purchasing new plants, look at the soil near the base of the plant and look for glossy larvae or flying insects.

Controlling fungus gnats must consider all stages of the insect from the flying adults to the soil larvae; you need to break the life cycle of the insect. For adults, yellow sticky cards is a great organic approach to reduce the population. Reducing the number of adults will cut back on eggs and therefore reduce the larvae. Since wet soil is this insect’s favorite environment, let the soil dry out. Larvae will thrive in the top layer of soil; so, keep the top two inches of soil dry. Try watering from the bottom instead of the top of the container.

There are systemic insecticides that can be applied to the container soil; however, these should be used as a last resort. A more environmentally friendly approach is to consider a complete repotting of the plant into a clean container using fresh quality potting mix. If using insecticides, always read the product label completely. Control of fungus gnats along with the name of your plant must be listed on the product’s label.

The New York State Integrated Pest Management website has more information regarding fungus gnats and how to control them; there are fact sheets as well as informative videos. Visit their website, here.

While you’re visiting the website, sign up for their “What’s Bugging You” first Friday event webinars. On each first Friday of the month, at noon, NYSIPM professionals present a 30-minute webinar “What’s Bugging You”. You can see the 2023 schedule for all the topics and sign up for anyone to receive the link to view these webinars by visiting their website here.

From understanding fungus gnats, to carpenter ants, to other insects, to how to recognize problems with fruit crops, there’s something for all home gardeners on the New York State Integrated Pest Management “What’s Bugging You” first Friday webinars.

Cornell Cooperative Extension Oneida County answers home and garden questions which can be emailed to homeandgarden@cornell.edu or call 315-736-3394, press 1 and ext. 333. Leave your question, name and phone number. Questions are answered weekdays, 8 a.m. to 4 p.m. Also, visit our website at http://cceoneida.com/ or phone 315-736-3394, press 1 and then ext. 100.