Fungus Gnats Plague Some Houseplants

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Have you ever noticed tiny gnats fluttering around some your houseplants? If you have you are not alone. The insect in question is called fungus gnats. These are small 1/8-inch long black flies, which are the adult stage of larvae living in the soil of your houseplants.

What are fungus gnats?
Fungus gnats are a very minor houseplant pest, but are a major household nuisance. Who likes to have insects flying around the home? No one, so that is why people contact our office for control. It is helpful to know a little more about their lifecycle so that understanding the method for control is easier.

Fungus gnat habitat
Fungus gnat’s larvae feed on algae, fungi and plant roots in the potting soil. They also prefer a moist environment in the soil, and the vast majority is found in the upper 3 inches of the pot. The larvae themselves are very small and worm-like, and are transparent with a black head. The good news is that neither the larvae nor adult mosquito-like insect bite humans. Basically they just bug the living heck out of us!

Controlling fungus gnats
The important step in controlling the fungus gnat is knowing its preferred environment. We normally see higher numbers in the winter months due to the warm indoor temperatures and simply the fact that we are home and indoors more often. Another factor is, plants tend to dry out more slowly in the winter and we tend to continue to water on the same schedule as summer, creating a moist soil media which is the perfect breeding ground for the gnats.

The first step to gaining control is to reduce water and let the plants thoroughly dry out. If you have a fungus gnat problem it is recommended not to water until at least the upper 3 inches, or more have completely dried out. This reduces the survivability of the eggs laid in the soil and the development of the larvae. It is also a good idea to remove all standing water in the saucer, as another precaution.

Chemical controls for fungus gnats
Normally this takes care of the problem. But the next step would involve chemical treatments. Treatments can be applied to reduce the number of adult gnats but this will not eliminate the problem, as they are soil borne before hatching. It helps, though, to decrease the number of adult gnats that can mate and lay eggs. Yellow sticky traps found at garden centers can be used around infected plants. These traps help capture adults, which reduces the mating population.

Soil drenches can be applied to help control the larvae and eggs. The most commonly available product to homeowners would be a houseplant insecticide which contains imidacloprid. It is often available as an additive to potting soil, spikes or a liquid drench.

Non-chemical controls for fungus gnats
Another alternative to using chemicals is to repot the plant. Many times fungus gnats become more of an issue as potting soil breaks down and holds more water as its air space is decreased. When repotting be sure to remove as much of the old potting soil as possible because it contains the eggs and larvae, which will prolong the problem.

One other old wives’ tale recommendation is to cover the surface of the pot with about 1 inch of sand. The theory here is that the adults cannot lay the eggs on the soil surface nor can the emerging larvae make it out of the soil. This is not an Extension recommendation but one passed down by wise gardeners.

If you have never experienced fungus gnats in your houseplants then you are truly lucky. But if your plants suffer from this insect then try these control strategies, as they should help do the trick.

Have questions? The Garden Hotline is staffed by trained EMG volunteers and Extension staff who will assist you with questions.

Phone: (913) 715-7050

Email: garden.help@jocogov.org