Overwatering of indoor plants leads to fungus gnat infestation

Question: We have two Norfolk pines in our living room. They seem to be quite healthy, but there are lots of tiny flies all over the dirt that's in the pot. Whenever you brush your hand against the plants, a cloud of them fly up off the plant. What are they and how do we get rid of them?

Answer: Fungus gnats are oh-so annoying. They're a classic example of a nuisance pest. The good news is that they don't cause significant harm to your plants; they're just a pain to have around.

There are two primary genera of fungus gnats here in Pennsylvania, Bradysia and Lycoriella. But, regardless of which group you have on your Norfolk pines, identifying and managing them is the same.

Mature gnats are tiny black flies that measure ¹⁄₈th of an inch and live for about two weeks. During this time, females lay eggs in soil fissures. The resulting translucent, minute larvae feed largely on the assorted fungi growing in the potting soil, though they can also feed on fine roots and plant debris.

In a few weeks, the larvae pupate into adults within the soil and the cycle continues with several generations occurring together at any given time. While individually fungus gnats are barely noticeable, in large numbers they are hard to miss.

Unfortunately, fungus gnats can set up shop in just about any indoor plant. Outdoor potted plants are less susceptible because the soil tends to dry out more often and natural predators help control their populations.

Because fungus gnat larvae feed primarily on fungi growing in the potting soil, these insects typically don't cause a significant amount of damage to infested plants.

Overwatering inevitably leads to a fungus gnat issue. Constantly damp soils promote fungal growth, which serves as an excellent food source for the larvae. Simply cut down on watering to solve most fungus gnat issues. Water infested houseplants deeply – but not frequently – and only when the soil is dry. Be sure the pot itself has good drainage and the saucer underneath doesn't hold standing water.

If changing your watering routine doesn't clear up the problem, repot your Norfolk pines with new, sterile potting soil, gently removing as much of the old soil as possible without decimating the roots.

You can also trap adult fungus gnats on yellow sticky cards placed vertically an inch or two above the soil surface. Another option is to remove the top half-inch of potting soil completely and replace it with a topdressing of a small gravel-like substance such as Gnat Nix™ or chicken grit.

And, if a reduction in watering and yellow stick cards still don't do the trick to get rid of fungus gnats, there are two biological control products that are highly effective. Beneficial nematodes (Steinernema feltiae) and Bti (Bacillus thuringiensis var. israelensis) can be applied to the soil as a drench or spray. The first is a microscopic nematode that burrows into the soil and consumes the gnat larvae. It provides fairly long-term control of fungus gnats. The second is a biological insecticide made from a strain of bacteria that kills the gnat larvae. One popular brand is named Gnatrol®.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. Her website is jessicawalliser.com. Send your gardening or landscaping questions to tribliving@tribweb.com or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.