David Cappaert, Bugwood.org

Veteran houseplant owners and novices to indoor gardening alike, have most likely, at one time or another, experienced the plague that is a fungus gnat infestation. These tiny insects begin their lifecycle in the potting media of houseplants that have been consistently overwatered. Once established, fungus gnats multiply quickly, with populations usually localized to a certain plant, or group of plants.

This insect is not known for causing excessive damage to the plants themselves, fungus gnats are more annoying and unsightly than they are damaging. However, the larvae feed on organic material found in the potting media, which can sometimes include the roots. Another reason root damage is less likely is because fungus gnat larvae reside in the top 2-3” of the soil. These insects are attracted to moisture as that is where they lay their eggs. It is not uncommon for fungus gnats to attempt to fly into your mouth. Anyone who has experienced this knows that fungus gnats are not something you want to leave unchecked in the home.

Fungus gnat larvae. Photo by Richard Lindquist
Overwatered potting soil, perfect for fungus gnats. Photo by C. Johnson

Controlling moisture is key to controlling fungus gnats. Populations are most abundant when there is an overabundance of moisture in the area, usually this is caused by overwatering. Over saturated media is the perfect egg laying habitat for this pest. Managing your irrigation goes a long way towards managing this pest, letting soils get as dry as possible before watering is a highly effective yet simple control method. It is important to note, this technique can be damaging to some plant species that may require consistent moisture in their growing media. If that is the case there are other options for control. Bottom watering just enough so that the bulk of the root ball is wet but the top of the soil remains dry is also effective. This can be tricky and requires some experimentation to get right.

A butterfly shaped sticky trap. Photo C. Johnson

A popular method for monitoring as well as controlling fungus gnats and other flying insects is the use of yellow sticky cards. Insects are attracted to the bright yellow color and are then entangled once they land on the trap. I use these in my houseplants even when I don’t have a sever problem as they let me know what insects are present just by looking at what’s stuck to the trap. Adding these traps to an already established infestation will help reduce the  breeding population of flying adults. There area also chemical controls available for this pest in the form of pellets which are placed on top of the potting media. Bacillus thuringensis var. Israeliensis is a strain of bacteria that is also effective at controlling gnats and can be applied to the soil via drenching. Ensure that any chemical controls being applied are labeled for fungus gnats and that you follow manufacturer’s directions for indoor home use.

Carl Johnson

UConn Home & Garden Education Center, 2021

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