Protecting Plants from Fungus Gnats

As you wait for the winter to thaw so you can get back to your garden, you may be paying more attention to your houseplants. And, it’s possible, dealing with pest issues.

One very common plant pest if the fungus gnat. Infestations of these small, flying insects can range in severity from just being a nuisance to actually damaging plants. 

Experts from the Kemper Center from Home Gardening provide the following advice on identifying these pests and managing them when their populations get out of hand.

What is a Fungus Gnat?

Fungus gnats are small, black flies that can be found in large numbers around indoor plants and in greenhouses. They can also be found in outdoor garden beds in warmer weather, but are not considered problematic outdoors because their populations are typically kept in check by predators and weather conditions. Fungus gnats begin life as tiny larvae that live in the top 2-3 inches of potting soil. They feed on decaying plant material, algae, fungus, and, when those food sources run out, plant roots. 

When They Become a Problem

If infestations get out of hand, the damage to the roots can stunt plant growth or cause leaf drop. The larvae pupate after two weeks of feeding and the adults live for about a week. Adult fungus gnats are poor fliers and often stay close to the infested plant but will also congregate near windows or other light sources. Although they look somewhat like tiny mosquitos, the adults do not bite or sting, and do no further damage to the plant. Female fungus gnats will lay up to 200 eggs on the surface of moist soil.

How to Prevent an Infestation

Fungus gnats can be one of the hardest plant pests to totally eradicate. Prevention is always going to be more effective than trying to rein in large infestations once they have already taken hold. Take the following steps to stop an infestation before it starts:

  • Quarantine new houseplants in a separate room for a week to make sure you are not introducing fungus gnats to your other plants.
  • Allow the top 1-2 inches of soil to dry out before watering. This will help reduce larvae numbers and discourage adult females from laying eggs on the soil surface.
  • Do not water houseplants the same amount in fall and winter as in spring and summer. The shorter days and lower temperatures mean that plants will be taking up less water. Overly moist soil provides fungus gnats with the perfect conditions to thrive.
  • Potting soil will break down over time and retain more water than fresh soil. Repot plants when the soil stops draining well.
  • Remove all plant debris from the soil surface. Decaying leaves and flowers provide the perfect habitat and food source for fungus gnats.

How to Manage an Infestation

Sometimes, infestations do happen. If you are dealing with an infestation in one of your houseplants, try the following to manage the problem:

• Use sticky traps to monitor for and reduce the numbers of adult fungus gnats.

• Chemical or biological controls may be required if infestations cannot be controlled by watering adjustments and sticky traps.

For more information about fungus gnats and managing infestations, view this guide from the Kemper Center for Home Gardening.

Catherine Martin
Public Information Officer

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