Small, dark flies hovering around potted plants signal an infestation of fungus gnats. These tiny insects, belonging to the Sciaridae family, are drawn to the consistently moist soil of indoor container gardens. While adult gnats are primarily a nuisance as they fly erratically, the real problem lies beneath the surface in the soil. Their larvae feed on organic matter and delicate plant roots, which can lead to stunted growth and decline in plant health. This guide provides a strategy for eradicating these pests permanently, addressing both the flying adults and the destructive larvae.
Identifying the Houseplant Pest
Confirming you are dealing with fungus gnats and not other similar pests, such as fruit flies, is the first step. Fungus gnats are slender, dark gray or black insects with long, dangling legs and a weak, low-flying pattern. They are often seen crawling across the soil surface and will briefly take flight if disturbed, but quickly settle back down. Fruit flies, by contrast, are typically tan or light brown with distinct red eyes and a more rounded body shape, preferring to hover around ripening fruit or kitchen waste.
The adults are harmless to the plant, living only a short time to reproduce. Female adults lay their eggs in the top inch of moist soil, and the resulting larvae are the destructive stage. These worm-like larvae with shiny black heads feed on fungi, organic matter, and the fine root hairs of plants. High larval populations can cause yellowing foliage, wilting, and significant root damage, especially in seedlings and young plants.
Quick Methods for Trapping Adult Gnats
Reducing adult numbers is an important step in breaking the life cycle, as they lay the next generation of eggs. Yellow sticky traps are highly effective because adult gnats are strongly attracted to the color yellow. Placing small, stake-style sticky traps near the soil surface will quickly capture flying adults, providing immediate visual confirmation and relief from the hovering insects.
Simple liquid traps can also catch a large number of adults within a few hours or days. A small dish filled with a mixture of apple cider vinegar, a few drops of liquid dish soap, and water works well. The vinegar attracts the gnats, and the dish soap breaks the surface tension of the liquid, causing the insects to fall in and drown. While these trapping methods are excellent for monitoring and reducing the flying population, they only treat the symptom and do not kill the larvae residing in the potting mix.
Soil Treatments to Eliminate Larvae
The most effective long-term solution requires treating the soil to eliminate the larvae, which are the source of the infestation. A common household remedy is a hydrogen peroxide drench, which targets the soft-bodied larvae on contact. Mix one part of standard 3% hydrogen peroxide with four parts of water. Pour this diluted solution over the soil until it is fully saturated and drains from the bottom. The peroxide reacts with organic matter, releasing oxygen that kills the larvae, then quickly breaks down into harmless water and oxygen.
Biological control uses the naturally occurring bacterium Bacillus thuringiensis israelensis (BTI). BTI is available in products, often sold as “Mosquito Bits,” and is selective, meaning it only affects the larvae of certain insects like gnats and mosquitoes. The BTI is ingested by the feeding larvae, releasing a protein crystal that destroys their gut lining. To use BTI, soak the granules in your watering can for at least 30 minutes, then use the infused water to water your plants. Because BTI only affects the larvae and not the eggs or pupae, repeat the application weekly for at least three weeks to ensure all newly hatched larvae are eliminated.
Food-grade diatomaceous earth (DE) can be applied to the dry topsoil as an effective surface barrier. DE is a fine powder made from the fossilized remains of diatoms, which are microscopic aquatic organisms. The powder works by physically abrading the outer layer of insects that crawl through it, leading to dehydration. Sprinkle a thin, uniform layer of dry DE over the entire soil surface to create a barrier that kills emerging adults and newly hatched larvae. Reapply the DE after watering, as it loses its effectiveness when wet.
Long-Term Strategies for Prevention
The most significant factor in a fungus gnat infestation is consistently wet soil, as the larvae require damp conditions to survive. Changing your watering habits is the primary long-term prevention strategy. Allow the top one to two inches of soil to dry out completely between waterings. This practice creates a dry zone that is inhospitable for adult gnats to lay eggs and for larvae to thrive.
Improving the drainage of houseplant containers is also crucial for prevention. Ensure all pots have functional drainage holes and avoid leaving plants sitting in saucers of excess water. Avoid over-fertilizing, as decaying organic matter provides a ready food source for the gnat larvae. When repotting, always use fresh, sterile potting mix to prevent introducing new eggs or larvae.