Fungus Gnats (Diptera: Mycetophilidae)
Fungus gnats, sometimes called root gnats, are fairly common insects that are generally found in moist, shady places. The larvae of most species live in moist or wet habitats and feed on fungi. Some have been reported as being occasional pests in mushroom cellars. Those found in Michigan homes commonly come from soil in potted plants. Another source is decaying vegetable matter, like an old bag of potatoes or some other forgotten, wet organic material. The best method of controlling the larvae in these situations is removing and discarding the infested material or to prevent a wet area from staying wet. Any household aerosol containing pyrethrins (e.g. Raid) should control the adults. Allowing potting soil to dry out a bit between waterings will usually reduce or possibly eliminate fungus gnat problems coming from houseplants. A more aggressive approach is to use an insecticide drench (i.e., use enough water with the insecticide to soak the soil) such as Sevin or Malathion to kill larvae in the soil. Plants must be taken outside before these materials can be legally used on houseplants indoors.
Be sure to read and follow all the instructions and safety precautions found on the pesticide label before using any pesticide.