GARDENER'S READING ROOM
Indoor Plants
Fertilizers, Pest Control and Soil Management

Fungus Gnat Wars

Know the enemy: Many species. Adults are small – 1/16 to 1/8 of an inch, with a few species measuring at 1/2 inch. Dark and delicate with wings dark grey to clear, they are weak fliers that generally hang out by potted plants, running with their wings flapping or resting on moist growing media, foliage, or plant litter or on the window. Often confused with shore flies, they don't bite people or animals but can be extremely annoying. A severe larval infestation (highly unlikely in the home; more likely in a greenhouse and in moist, warm outdoor conditions especially where there is a lot of decaying vegetation and standing water which are favorable to proliferation of many generations), can cause serious plant damage.

There are 4 growth stages: egg, larva (with 4 instars or developmental stages), pupa and adult. Adults live approximately 7 to 10 days. Females deposit 100 to 150 eggs into the soil in strings of 3 to 40. These hatch in approximately 4 to 7 days. Larvae, 1/4" white with shiny black or brown heads, feed on fungi and decaying organic matter. If food supply is exhausted and/or population becomes very high, larvae begin to feed on fine root hairs or other tender tissue. They feed for about a month before pupating in the soil. This is a rest stage. Adults emerge in approximately 3 to 7 days. Many generations and growth stages can exist at the same time.

Symptoms: Adults as described above. Plants with severe soil infestation (again, very unlikely in home conditions) will have stunted growth, lack vigor, and may have yellowing leaves. NOTE: These symptoms are also present with other plant problems.

Plan of attack: Larvae are difficult for the homeowner to control, as the most effective remedies are only available commercially. So, let's concentrate on the adult.

Goal: To manage adults so you won't have to worry about larvae. As soon as you see activity, place a yellow sticky trap stick in the pot. Discourage adults from flying into soil by placing grit or small pebbles on soil to a depth of about 1/3". Cover soil completely. (Sand gets messy and some of it sinks after a while.) Adults can be killed with pyrethrin or any spray labeled for "gnats" or "flying insects." Moistened soil can also be sprayed with pryethrin to kill any lurking adults. Sundew types of carnivorous plants are good living sticky traps, but should be used in conjunction with the yellow sticky trap.

Water properly for the type of plant you have. Do not create soggy soil conditions. Conversely, too little water may cause the larvae to enter plant stems in search of moisture. Clean plant litter from top of, and around, pots. Groom your plants. Don't leave water standing in the saucer or other surfaces. Don't leave unused potting soil bags open and where they can stay moist. Gnats will fly in and breed.

Please give a call if you have any questions or need further help.

By Mary Ann Greco
Skylights Spring 2009, Vol 24, No. 1

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Other articles on fertilizers, pest control and soil management