Discard the soil immediately, and replace this top layer with sand, which is free of organic matter and dries out quickly—an unappealing medium for fungus-gnat eggs. Let the soil below dry out completely before the next watering.
Properly discard the top 2 inches of soil and replace it with fresh, sterile potting mix to get rid of any remaining fungus gnat eggs and larvae. If desired, add a 1/2-inch layer of sand or diatomaceous earth to keep new infestations from occurring.
Be ruthless–and repot infected plants.
Fungus gnats can spread from one plant to another fairly quickly, so take caution. If the infestation isn't so bad, consider repotting the plant.
Boiling water: Boil water on your stovetop. Transfer the soil to a bowl and then pour the boiled water into the bowl slowly and carefully. The high water temps (212 degrees) kill adult fungus gnats and their eggs.
How to Prevent Fungus Gnats. Neither fungus gnats nor their larvae can survive in dry soil, so let your soil dry out completely between watering as often as possible.
Hydrogen Peroxide
A peroxide solution apparently kills larvae and eggs on contact, effectively killing off all infant gnats and disrupting the lifecycle. You should be able to buy hydrogen peroxide from any high street pharmacy or online. Mix 1 part 9% hydrogen peroxide with 6 parts water.
The larvae feed for about 2 weeks and then pupate near the soil surface within thread chambers. After 3 to 7 days in the pupal stage, adults emerge and live for about 8 days. The gnats develop from egg to adult in 3 to 4 weeks. Their life cycle is dependent on temperature.
Adult fungus gnats don't damage plants or bite people; their presence is primarily considered a nuisance. Larvae, however, when present in large numbers, can damage roots and stunt plant growth, particularly in seedlings and young plants.
Fungus Gnat Deterrents
Fast-draining soils with inorganic additives, like perlite, vermiculite, and sand; Inorganic soil covers, like pebbles, coarse sand, chicken grit, or diatomaceous earth (Some swear by the gnat-killing power of diatomaceous earth!); Pyrethrin sprays approved for organic gardening.
A fungus gnat infestation can come from anywhere. The most common ways fungus gnats get into your house are either they're in the soil of a newly purchased plant, or in a bag of potting mix that you bring indoors.
If you've got fungus gnats, you most likely also have pythium root rot, as gnats can carry it. Yay, fungus gnats and root destroying bacteria! Phytophthora and Fusarium are fungi found naturally in the soil; there are several different species.
Fungus gnats are also capable of spreading the plant pathogen that causes damping off and the eventual death of seedlings. Fungus gnat damage will appear similar to that of any other root-related issue, such as root rot. Lower leaves may turn yellow and drop, and the plant's growth may slow down or stop completely.
Attracted to damp soil, fungus gnats usually get into homes by flying inside or from infested houseplants brought inside the house. Fungus gnats infest soil and feed upon decaying or damaged plant roots as well as fungi found in the soil.
Mix one part of hydrogen peroxide and four parts of water. Use this solution to water your plants. Hydrogen peroxide will kill the gnat larvae and not injure your plant. Keep reapplying weekly until you notice the gnats are gone.
Gnats, especially fungus gnats, are attracted to damp soil and that's where they lay their eggs. If you're overwatering your plants, these bugs will find their way there and start laying eggs. They can lay up to 800 eggs in one breeding cycle so this can become a major problem very quickly.
“These pests are attracted to moisture, so you may see swarms in areas with damp soils, standing water, or even houseplants that have been overwatered,” Ludwiczak adds. “The breeding habits of gnats are most active during warmer months, which is why they seem to be especially prevalent during summer.”
Repel them with Scent.
Or, you can switch up your favorite scent with something they don't like. Try Lighting citronella candles, using lemon or vanilla sprays. While gnats are a fan of sweet-smelling fruit, they can't seem to stand vanilla, lemon, or even lavender. A little spritz can at least keep them at bay.
Gnats prefer to lay their eggs in damp soil filled with decaying organic matter, which is why they love hanging out in greenhouses and around houseplants. African violets, poinsettias, and geraniums are especially prone to injury caused by fungus gnat larvae feeding on roots.
The thing about fungus gnats is that you have to stop them at every stage of the life cycle to eradicate them. If you kill only the adults flying around by trapping them, some will get the chance to lay another round of eggs in the soil, and the cycle begins anew.
Fungus gnat females lay small, yellowish-white eggs on the surface of moist soil. The larvae that hatch are legless, with white or transparent bodies and shiny black heads. Fully grown, larvae measure approximately 1/4 inch in length.
Fungus gnats go through complete metamorphosis (egg, larva, pupa, adult) and reproduce in moist, shaded areas within decaying matter. One generation takes about 4 weeks. Mated females lay 00 to 200 microscopic eggs. Eggs are oval, smooth, shiny white and semi-transparent, and hatch in 4 to 6 days.
Bleach Your Drains
If you notice gnats around your kitchen sink or bathtub, pour a cup of diluted bleach down the drain to kill the gnats. Be sure to pour the cup of bleach down each drain or garbage disposal very slowly until there are no more gnats in your line of sight.
Unlike hydrogen peroxide, neem oil solutions kill both larvae and fungus gnats and thereby speed up the process of eliminating the pests for good. To create a neem oil spray, simply mix a teaspoon of dish detergent with a gallon of water and two tablespoons of neem oil.
Additionally, hydrogen peroxide acts as an insecticide, killing the fungus gnats on contact. To get rid of fungus gnats, mix a solution of one part hydrogen peroxide with four parts water. Pour the mixture onto the soil around the infested plant.