Answer: Fungus gnats look like tiny fruit flies and will occasionally infest the soil of houseplants, especially when plants are kept evenly wet. They breed in damp soil, so the first line of defense is simply to allow the soil surface to dry out a bit between waterings. (Your plants would probably appreciate this as well.) An alternative would be to cover the soil with a thin layer of fast draining grit or sand which would dry almost immediately after watering. By breaking the breeding cycle they should disappear in about ten days to two weeks. You could also use a soil drench called Gnatrol according to the label instructions for immediate results.
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