Fungus Gnats may be the smallest live creatures you will ever see without the aid of magnification. It's amazing that they have knees that bend and wings that fly, and the ability to locate damp humus to lay their eggs.
You may notice the dancing activity of newly-emerged winged adults in flower pots attempting to attract a mate, which prefaces egg-laying activity. This is a good time to introduce them to an Insecticide made for "Flying insects".
Bottom drainage openings in pots have conditions they prefer for their activity. When eggs hatch, the larvae feed on humus and rootlets. An infestation of them can stunt the growth of tiny seedlings and even larger plants.
Using potting soils which have been sterilized by heating improves your chances of avoiding Fungus Gnats. The next-to-invisible eggs should be destroyed in the process. But putting one pot outdoors may be an invitation for an egg-laying female to start a population that may later be taken indoors to infest your other plants.
To keep them out of potted plants means isolating the damp humus that you may be using to nourish plants. Mulching your pots with a half inch of dry material like sand or seed-starting mix may help. A layer of plastic under this mulch will also help, but then watering may have to be done from the bottom.
Using a paper towel in the bottom of pots to cover the drainage holes before adding soil will help to keep the soil in and the Gnats out. Overwatering is an attraction to these pests. Allowing plants to dry out to the point of drooping creates a condition that the Gnats find intolerable, and may be your first line of defense.
Malathion insecticide sprayed on the mulch and around drainage holes may limit their activity, but to reach the destructive Larvae in the area of roots, you may have to resort to an occasional dunking in a Malathion solution. Take the plants outdoors to treat them and follow the package directions.
They are attracted to light which may be to your advantage if you rig up a Gnat catcher using Petroleum Jelly smeared on the outside of a glass jar which contains a tiny night light. It may be a battery-operated light or one that plugs into an extension cord as shown above. Even a candle may serve the purpose, if there is not so much heat generated that the Petroleum Jelly runs down the jar.
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