The fungus gnat is a bit of a pest. Not because it bites people, but because in large numbers these flies can damage the roots and stunt the growth of seedling plants. The fungus gnat larva's diet of fungi and organic matter in soil makes it a bigger nuisance than the adult gnat, who generally feeds on flower nectar.
Adult fungus gnats lay their eggs on fungus or in soil. When the larvae emerge from the eggs, they burrow into the material they were born on. This could be a wild mushroom, or it could be a house plant root. The larvae feed on the insides of the mushroom for around two weeks, then they crawl out and go into the pupal stage. The adults appear about four days later. It is during the larval stage that most damage is caused to greenhouse and indoor plants. Eggs laid on seedling roots eat the roots during the larval stage, which destroys the plant. These gnats don't cause the same amount of damage through eating to outdoor plants, according to the University of California's Integrated Pest Management Program.
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