Controlling Fungus Gnats When You're Growing Indoors
In severe infestations, the larva can damage tender feeder roots (they're in the soil mostly eating soil algae but can snack on small roots as well).
Controlling Fungus Gnats
There are several ways to control this pest on indoor plants. Note you rarely see this problem in the garden as natural predators control this pest outdoors.
The first is to understand the main problem is a lot of soil algae as a food source and the main culprit here is overwatering. Too much water produces algae and this feeds the pest. Water well and then allow the pot to dry down between waterings. This will be your long term method of controlling them.
However, in the short term you have a bunch of tiny, annoying black flies buzzing about.
You can spray the adults with insecticidal soap. Spray under leaves as well as tops for best control.
You can hang sticky yellow cards around the plant - these are available from garden shops and this pest loves these cards. You'll collect a lot of them.
Controlling the adults will eventually kill off the larva.
For the larval forms of fungus gnats, you can heavily spray the surface of the pot with insecticidal soap. This drench will kill off the larval forms.
You can put a half inch layer of sand on top of the soil in the pot. This will tend to dry out quickly and the fungus gnats will not survive in this dried out top layer. Once they are controlled, you can remove the sand.
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Fungus gnat control sprays and sticky traps - note some non-organic products have crept into this area as well so avoid them