Kill fungus gnats effectively by setting sticky traps for the adults and using insecticide on the larvae. Keep them away by letting the soil dry before watering plants, putting screens on windows and using quality potting mixes.
- Identify the fungus gnats
To be sure the tiny insects hovering over the plants are fungus gnats, place a slice of raw potato in the soil. Within hours, fungus gnat larvae migrate to the potato and are easily seen.
- Set sticky traps
Place yellow sticky cards around the edges of infested containers and across the potting soil. Adult gnats are drawn to the color yellow and become trapped on the cards. Although this may not kill all of the adults, it decreases the amount of larvae in the following generation.
- Use insecticide
Killing adult fungus gnats does not dispose of the larvae, which cause severe damage in greenhouses by eating plant roots. To kill the larvae, drench the infected soil with bacillus thuringiensis, a liquid microbial insecticide, or use granules or sprays containing imidacloprid.
- Prevent a re-infestation
Once the fungus gnats are exterminated, keep them away by placing screens on greenhouse or home windows and vents. Because the larvae cannot live without moisture, allow topsoil to dry completely between watering sessions. When re-potting plants, change the growing medium. Be sure to use only pasteurized, well-draining potting mixes.