Fungus gnats are small, dark, mosquito-like flies often seen hovering near the soil surface of indoor plants. While the adults are primarily a nuisance, the true problem lies beneath the soil. The tiny, translucent larvae feed on organic matter, including delicate plant roots and root hairs. This feeding can lead to stunted growth and wilting, especially in seedlings and young plants. Eliminating an infestation requires managing the flying adults and eradicating the root-feeding larvae.
Immediate Spray Solutions for Adult Gnats
While the larvae cause the damage, tackling the flying adult population is necessary to stop the reproductive cycle. Female gnats lay hundreds of eggs in the moist top layer of soil, so spray solutions provide an immediate contact kill for the adults.
Insecticidal soap is a highly effective option, using potassium salts of fatty acids to break down the insect’s outer layer upon contact. Thoroughly spray the adults and the top layer of soil with a diluted solution, typically following product instructions (often 1% to 2% active ingredient). Since the soap has no residual effect once dry, repeated applications every few days are necessary to catch newly emerged adults.
Another popular surface application is neem oil, an organic pesticide derived from neem tree seeds. Neem oil works as a contact insecticide for adults. When mixed with an emulsifier, such as a few drops of dish soap, it can be diluted with water and sprayed onto the soil surface. This application disrupts the life cycle by interfering with the gnat’s hormonal system and acts as a mild deterrent for egg-laying adults.
For a fast, localized spot treatment, a highly diluted rubbing alcohol solution can directly spray adult gnats. A common dilution is mixing one part 70% isopropyl alcohol with three to four parts water. Always test a small area of the plant foliage first, as alcohol can damage sensitive leaves. Never apply this solution under direct sunlight or extreme heat. These contact sprays provide immediate control but must be paired with a soil treatment for complete eradication.
Targeting Larvae: Soil Drenches for Eradication
Permanent control requires eliminating the larvae living in the top two to three inches of the potting mix, which is best achieved through a soil drench. One specific treatment involves using the naturally occurring soil bacterium, Bacillus thuringiensis israelensis (BTI). BTI produces a toxin activated only in the highly alkaline digestive systems of certain insect larvae, including fungus gnats and mosquitoes.
The BTI product is mixed with water, often called “BTI tea,” and applied as a thorough soil drench during a regular watering cycle. When gnat larvae ingest the bacteria, their feeding stops almost immediately, and they die within a few days, effectively breaking the reproductive cycle. BTI is non-toxic to humans, pets, and beneficial organisms like earthworms and ladybugs, making it a preferred biological control method.
A temporary, non-biological method involves drenching the soil with a diluted hydrogen peroxide solution, which kills larvae and eggs on contact through rapid oxidation. Standard household 3% hydrogen peroxide is mixed with water at a ratio of one part peroxide to four parts water. When poured onto the soil, the fizzing action indicates the peroxide is reacting with organic matter, including the soft-bodied larvae.
This method quickly oxygenates the soil but can disrupt beneficial microorganisms. Therefore, it is recommended as an initial knock-down treatment for severe infestations. For a sustained biological approach, beneficial nematodes of the species Steinernema feltiae can be applied as a soil drench. These microscopic, parasitic roundworms actively hunt down gnat larvae, entering their bodies and releasing a symbiotic bacterium that kills the host. The nematodes then reproduce inside the dead larva, providing a longer-term, self-sustaining control measure.
Preventing Recurrence Through Watering and Trapping
Addressing the environmental conditions that attract fungus gnats is the most effective long-term preventative strategy. Fungus gnats thrive in overly moist soil, which encourages the growth of fungi and decaying organic matter their larvae feed on. The most impactful change is adjusting watering habits to allow the top one to two inches of potting mix to dry out completely between waterings.
Allowing the soil surface to dry creates an inhospitable environment, killing existing eggs and deterring adult females from laying new ones. This practice also strengthens the plant by encouraging a deeper, more robust root system. If a plant requires consistently moist conditions, bottom-watering can be implemented to keep lower roots hydrated while allowing the top layer of soil to remain dry.
To physically manage the adult population and monitor the infestation level, yellow sticky traps are recommended. Fungus gnats are strongly attracted to the color yellow, which mimics tender, new growth. Placing small, stake-style sticky traps directly into the soil or near the plant effectively captures flying adults. This prevents them from laying eggs and helps gauge the severity of the problem.
Another useful physical barrier is applying a thin layer of fine-grade horticultural sand or diatomaceous earth (DE) across the potting mix surface. Diatomaceous earth is a natural powder made from the fossilized remains of diatoms. Its microscopic, sharp edges kill larvae and adults that crawl across it by causing desiccation. This barrier makes the soil surface less accessible and less appealing for egg-laying, helping to break the life cycle.