How to Deal With Annoying Fungus Gnats
Copyright 2011 McGroarty Enterprises Inc.

Fungus gnats are tiny flying insects that can be a problem for houseplants and greenhouse plants. Not only are fungus gnats annoying as they fly around, but they can also do some serious damage to plants.

Fungus gnats are mosquito-like insects, about an eighth of an inch long with one pair of transparent wings. The adults will scatter and fly about when the host plant is disturbed, which is often the first indication of an infestation. Adult fungus gnats are harmless except for their egg-laying capabilities. They do not feed on plants, nor will they sting or bite people or pets. 

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It is the larvae of the fungus gnats that do the most damage. The adults live about a week, but during that time they lay hundreds of eggs in the soil of your potted plants. They are particularly attracted to damp soil that is rich in organic matter. Fungus gnat eggs hatch in four to six days, and for about the next two weeks the larvae feed on plant roots and root hairs in the soil. This can weaken the plants, stunting their growth and causing sudden wilting or yellowing of the leaves. In severe infestations plants may drop their leaves and new growth can be disfigured.

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If you see fungus gnats flying around your plants, you can be sure that there are also larvae feeding on the roots of your plants. To monitor the fungus gnat population, use yellow sticky traps placed horizontally near the plants. Yellow sticky traps are available at many garden centers. Adult fungus gnats have poor flying skills, and as they flit about they will become stuck to the yellow sticky traps. The yellow sticky traps should be used only to monitor the population and should not be considered a means of controlling the population. They will not trap the larvae which are the real culprits.

There are methods to kill fungus gnat larvae before they kill your plants, but first you must determine whether the flying insects are truly fungus gnats, or simply fruit flies. 

If you suspect a plant is infested with fungus gnat larvae, you can tip the plant out of its pot and use a magnifying glass to examine the top inch or two of its soil. If fungus gnat larvae are present, you should be able to see the whitish or clear thread-like larvae near the soil surface.

You can also place inch-long pieces of raw potato on the soil surface. If fungus gnat larvae are present, they will migrate to the potato within four hours for a free lunch. Look for the tiny black-headed larvae both under and on the potato piece, then dispose of the potato.

Once you know that fungus gnats are present, the adults can be killed with any insecticide spray labeled for gnats or flying insects. Treating the larvae in the soil is another matter. A soil drench is most effective in killing the larvae. There are products available specifically for killing fungus gnat larvae, including a form of Bt (Bacillus Thuringiensis Berliner var. israelensis). Gnatrol is the brand name of one form of Bt specifically for fungus gnats. There are also parasitic nematodes that attack fungus gnat larvae, although this product is more useful for large-scale plantings. Neem oil is also an effective organic control for fungus gnats and can be used as a soil drench to kill the larvae.

Fungus gnats can also be easily controlled with a simple product you may already have at home. Fungus gnat larvae will die on contact with hydrogen peroxide. Be sure to use the ordinary 3% solution hydrogen peroxide with no added ingredients. First, allow the soil to dry out for a few days so the top two inches of soil is dry. This is where the larvae of fungus gnats reside, and they cannot survive in dry soil. During times of drought, the larvae will actually suspend their development, but once the soil is moist again, they will resume growing and eating. 

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Once the top layer of soil is dry, mix one part hydrogen peroxide with four parts water. Using this solution, water your plants as you normally would. The fungus gnat larvae will die on contact with the hydrogen peroxide, but the solution will not hurt your plants so long as it is mixed correctly. It would take an awful lot of hydrogen peroxide to harm the plants. When the hydrogen peroxide is watered in it will fizz a bit, but in a very short time it will naturally break down into nothing more than oxygen and water molecules which the plant will love. 

Continue to monitor the fungus gnat population with yellow sticky traps, and reapply the hydrogen peroxide drench weekly as long as you continue to find adult fungus gnats buzzing around your plants. As extra insurance against the gnats, you can water your plants with the hydrogen peroxide mixture once a month to prevent further outbreaks of this little pest.

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by Michael J. McGroarty
Copyright 2011