Fungus Gnats in February?

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In the house, yes it’s possible. The flying version is annoying, but it’s the larvae that feed on the roots of my precious houseplants, as well as on other fungi in the organic-rich soil that cause my concern. They are a particular pest in greenhouses in winter and spring as well. Adults live about 1 week, during which time each female deposits 100 to 150 eggs. They are laid in strings of 3 to 40 on soil, usually near stems of plants. With indoor infestations, there will typically be continuous reproduction and overlapping generations with all life stages likely to be found throughout the year.

While their food source is organic matter and they’re most often found where there Gnatare over watered houseplants, houseplants aren’t the only source of food for them. So don’t pitch out all the houseplants that you are overwintering inside just yet.

Other locations include floor drains that are relatively unused and in the ductwork of certain kinds of AC units in a home. They like high humidity and cooler temperatures. Places that fit the bill could be houseplants sitting on a cool windowsill, a downstairs/basement bathroom, or other drain or ductwork that might not get used as frequently at this time of year. Do you have a sump pump pit or another plumbing issue that isn’t working properly?

Too often we think we can “spray away the problem” when we really can’t be successful with that strategy long term. The key to solving indoor fungus gnat problems is to find and eliminate the source, i.e., find the area(s) of excess moisture.

Tips to Control the situation:

  • Check plants to see if the soil is excessively wet. Drain any excess water from the dish/container below flower pots. If the weather permits, move the plants outdoors or allow the soil to dry (not to the point where plants will wilt). Then, simply add a day (or more) between regular watering and the problem should improve.
  • Check areas where moisture is commonly found: kitchens, bathrooms, utility rooms (e.g., a leak in the waterline to a clothes washer), as well as crawlspaces.
  • Check other areas where leaks may occur as a result of storm damage or poor maintenance, e.g., roofs, crawlspaces, and walls and floors near exterior walls. Water leaking through a break in the roof (whether caused by damage or improper sealing) will result in the underlying insulation becoming wet and possibly moldy creating an ideal habitat for fungus gnats. Inspect the roof and ceiling below for signs of leaks.
  • Try to determine what areas of the building have the highest numbers of gnats. Check window ledges, light fixtures, etc. Fungus gnats are mobile and are more likely to move towards a nearby source of light. Pest control companies use Insect light traps to help determine which areas are most heavily infested.
  • Correcting a moisture problem and cleaning the area should eliminate the fungus gnats fairly quickly.

For more information about this nuisance insect, browse these publications:

Fungus Gnats Indoors and this publication from our colleagues at the University of Maryland, Fungus Gnats – Indoor Plants.

Minda Daughtry is the Horticulture Agent for North Carolina Cooperative Extension in Lee County.