Fungus gnats can become a nuisance indoors when adults emerge in large numbers from potted plants containing consistently damp or wet soil. While the adults are harmless to humans and animals, the eggs they lay become larvae or maggots, and can damage plants.
African violets, carnations, cyclamens, geraniums, poinsettias and indoor foliage plants can be susceptible and show symptoms of sudden wilting, loss of vigor, poor growth, or leaf yellowing and foliage loss. The larvae feed not only on fungi and decaying organic matter, but on living plant tissue, particularly root hairs and small feeder roots.
Overwatering is the usual cause of fungus gnats, so it is important to focus attention there. It’s best to allow the soil to dry as much as possible, without injury to the plants, as an effective natural way of controlling them. Another natural solution is to cover the soil with a one-inch layer of decorative rock, gravel, or sand.
An initial infestation can easily be controlled with an application of a pyrethrin spray. It helps to do at least one follow-up application 7-10 days later to break the breeding cycle.