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Fungus Gnats

By Judy Sedbrook, Colorado Master GardenerSM, Colorado State University Cooperative Extension, Denver County

Nearly everyone who grows houseplants has at some time experienced the annoyance of fungus gnats.

These tiny black flies suddenly appear in large numbers and are often first noticed by windows as they are drawn to the light. They inhabit the fungus and moist, decaying plant material that can often be found at the base of houseplants.

Adult fungus gnats are harmless to animals and humans and they do not feed on or damage living plant tissue. The larvae, or maggots, feed on all organic matter in the soil. This includes the feeder roots and root hairs necessary to sustain a plant. When these delicate roots are damaged, the plant may wilt, loose vigor, and have poor growth. The foliage turns yellow and the plant drops leaves. Fungus gnats can cause serious damage to African violets, cyclamen, geraniums, foliage plants, and poinsettias.

Prevention

Carefully examine new plants for any signs of insect infestation before purchasing. If you put your houseplants outdoors in the summer, be sure they are insect-free before bringing them back into the house. Always use a sterile potting soil mix when planting or re-potting. Take care not to overwater your plants, as fungus gnats require a moist environment to feed and breed. Be sure that the container you are using has good drainage and allow the soil to dry out between waterings.

Control

Using good cultural practices is the best way to control fungus gnats. By allowing the soil to dry as much as possible, without injury to the plants, you can effectively kill many maggots. Plants should be repotted in a sterile potting mix with a lower percentage of organic matter. Practice good sanitation. Remove dead plant material and debris from the garden as well as houseplants. Place yellow sticky traps near susceptible plants to monitor the gnat population and trap flying adults. Check the traps every 3 or 4 days as eggs hatch every 4 to 6 days and there are many overlapping generations throughout the year. Replace when they become covered with insects.

Insecticides

Adult fungus gnats can be easily killed using pyrthrins or pyrethroids. The treatment will need to be repeated every few days for at least 2 weeks for adequate control. Bacillus Thuringiensis Berliner var. israelensis (Gnatrol, Vectobac) can be used to control the infestation in the larval stage. The larval stage is best controlled by treating the soil, using a soil drench. Wet the soil surface thoroughly. Repeat weekly or as needed. Treat plants outdoors, allowing them to air out and dry during the day before bringing them back inside. Whenever using any type of pesticide, be sure to read the label carefully and follow directions and safety precautions.

Photograph courtesy of Judy Sedbrook.

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Date last revised: 01/05/2010