Other methods for controlling fungus gnats include drenching the soil annually in an insecticidal soap.Since the gnats are weak fliers, fan-based traps as well as other fly-killing devices may be used to help control free-flying gnats, especially indoors.
Note that fungus gnat larvae wont be affected by this treatment method, so the insecticide will need to be reapplied frequently to be effective. These instructions generally apply to using pyrethrin to get rid of fungus gnats, but defer to the manufacturers instructions in all cases.
No, Fungus Gnats do not kill houseplants as they only feed on the organic matter of potting mix soils. Here's an example of what the Fungus Gnat larvae looks like in a houseplant's potting mix - small, white wiggly larvae that'll inhabit the top 2-3cm of the soil.

Furthermore, visual representations like the one above help us fully grasp the concept of Soil-Based Methods For Killing Fungus Gnats.
Fungus gnat larva, likely Keroplatidae family. The life cycle of the fungus gnat is made up of four stages: egg, larvae, pupae, and adult. Adult fungus gnats lay about three hundred eggs in the moist, rich soil at the base of the plant during their short lifespan.
In this video I will give you 10 organic gardening solutions for preventing and killing fungus gnats.

Quick Fixes. 1. Drying Out Soil. Many infestations happen as a result of overwatering plants. By waiting longer between watering sessions, theoretically you can make your plant soil dry and inhospitable to gnats. Method. Turn up around 2cms of soil in the plant.
Method Solution of hydrogen peroxide used to kill fungus gnats. Mix 1 part 9% hydrogen peroxide with 6 parts water. Allow top layer of soil to completely dry out before drenching the soil in the peroxide solution.

This particular example perfectly highlights why Soil-Based Methods For Killing Fungus Gnats is so captivating.
Fungus gnats signal soggy soil that's killing your citrus roots.Mineral-based soil prevents both fungus gnats and root rot permanently. Safe, approved treatments include diatomaceous earth, castile soap, and rubbing alcohol.