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The Garden Plot for February 15, 2008
3:55 PM - 4:00 PM
[Program Website]
Today's Highlight: “Fungus Gnats”
If you raise plants indoors, you are probably familiar with the tiny, dark, fragile-looking flies that flit through foliage and congregate on windows, during cloudy weather, in the winter. Theses flies are called “fungus gnats”. Their primary food is the organic matter and fungus found in potting soil or planting medium. Unfortunately, fungus gnat larvae also feed on plant roots and can become a serious problem in greenhouses where bedding plants and vegetables are grown. Geraniums, ornamental peppers, and poinsettas are among the plants most commonly attacked by fungus gnat larva.
Adult fungus gnats like moist areas. They live about one week and can lay up to 200 eggs. The females are attracted by soils and planting mixes that have a high organic matter content. They particularly like composted manures and wood bark. Fungus gnat adults lay their eggs on top of the soil near plant stems. Eggs hatch in about 4 days into tiny larva that are clear with black heads. Larva feed on root hairs and roots in the upper inch of soil. Later they can be found deeper into the soil and inside stems. Young plants with small, developing root systems are most susceptible to fungus gnat larva damage. Injured plants will first show signs of wilting. On closer examination of the roots on wilted plants, you will see stunted roots with little branching and few root hairs.
You can monitor for fungus gnat adults using yellow sticky card traps. In fact, if you are growing only a small number of plants, yellow sticky cards may effectively control fungus gnats. Place sticky card traps near the base of plants.
If you catch a large number of fungus gnats each week, for several weeks in a row, you may want to manage them. For commercial greenhouse growers there are two very effective least-toxic control options. The first is Bacillus thuringensis, israelensis strain. This is a Bt specifically for the larva of flies. It is ineffective on adult fungus gnats. Mix the Bt – israelensis with water and drench the soil. Repeat applications may be necessary. The trade name for Bacillus thuringensis, israelensis strain is Gnatrol. Gnatrol can be used by homeowners, but it only seems to be available in large 2 1/2 gallon containers. Because Bt is a biological control, it does not have a long shelf life. It should not be stored for more than 12 months.
Another effective least-toxic control for commercial greenhouse growers is an insect growth regulator called Methoprene. Methoprene also acts only on the larva. It can be used with yellow sticky traps to catch adult fungus gnats.
Other least-toxic fungus gnat management options for homeowners include insecticidal soap or Pyrethrin/Rotenone sprays on foliage and on the potted plant’s soil surface. This will reduce adult fungus gnats. If you can keep the top inch of the soil dry, it will diminish the number of fungus gnat eggs that hatch and survive.
Another management treatment is to grow a “trap crop” of sprouted wheat. Sow wheat seed into a soil media that contains composted manure. Keep the soil in the sprouted wheat pot moist. This will encourage fungus gnat adults to lay eggs in it.. Let the seed sprout and grow for a week, near your house plants. After a week or two, discard the pot outside and start a new pot of sprouted wheat. It will take several weeks to clear up a fungus gnat infestation using this trap crop method.
For more information on fungus gnats and other indoor insect pests, ask your local county Extension agent.
Helen Atthowe's new short program of gardening tips
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