Small, dark flies hovering around indoor plants are a common source of frustration for houseplant enthusiasts. These tiny insects are typically a sign of an imbalance in the growing environment, often related to moisture levels in the potting soil. While they are mostly a nuisance pest, a large infestation can cause damage to plant roots and spread fungal diseases. Successfully eradicating these pests requires a multi-pronged approach that targets both the flying adults and the developing young in the soil.
Identifying the Culprit: Fungus Gnats
The small flies seen near houseplants are almost always fungus gnats, belonging to the family Sciaridae. Adult fungus gnats are delicate, dark-colored insects, typically measuring between 3 and 4 millimeters long. They are weak fliers, often observed resting on the soil surface or flying in erratic, short bursts when disturbed. They have long, slender legs and segmented antennae that are longer than their head.
The fungus gnat life cycle lasts about three to four weeks, depending on temperature and humidity. Females lay up to 150 tiny eggs in the moist top layer of the potting mix, which hatch into larvae. The larvae are translucent or whitish with a distinct shiny black head capsule. While they feed primarily on organic matter, high numbers will chew on fine plant roots, leading to poor growth or yellowing of the foliage.
Immediate Action: Trapping Adult Flies
The flying adults are the reproductive stage, and trapping them immediately helps slow population growth by preventing egg-laying. Targeting the flying population provides visible relief while longer-term soil treatments take effect. Yellow sticky traps are highly effective because fungus gnats are naturally attracted to the bright yellow color. Place these traps horizontally or vertically near the soil surface, where adults tend to congregate and fly.
The adult gnats are lured to the color and become permanently stuck to the adhesive surface. However, reducing the adult population count alone will not eliminate the infestation, as it does not affect the larvae developing in the soil.
A simple DIY liquid trap can also be used to capture flying adults. A mixture of apple cider vinegar, a few drops of dish soap, and water attracts the gnats with its fermenting scent. The dish soap breaks the surface tension of the liquid, causing the attracted insects to sink and drown. Place these shallow bowls near infested plants to supplement the sticky traps.
Eliminating Larvae: Targeting the Soil
Eliminating the larvae is the primary step because they are the source of the infestation and the stage that damages plant roots. Since the larvae live in the top few inches of moist soil, treatments must be applied as a soil drench to reach them. Two effective methods for targeting larvae are a hydrogen peroxide drench and the application of a biological control agent.
Hydrogen Peroxide Drench
A hydrogen peroxide drench works as a quick-acting, temporary insecticide that kills soft-bodied larvae on contact. A common household 3% hydrogen peroxide solution should be diluted with water, using a ratio of one part peroxide to four parts water. When the mixture is poured onto the soil, the peroxide reacts with organic matter, releasing oxygen that fizzes and instantly kills the gnat larvae. The peroxide quickly breaks down into harmless water and oxygen.
Biological Control: BTI
For a long-term biological solution, products containing Bacillus thuringiensis israelensis (BTI) are recommended. BTI is a naturally occurring bacterium that produces protein toxins lethal only to the larvae of specific insects, including fungus gnats. The gnat larvae ingest the BTI spores and toxins while feeding, which disrupts their gut cells and causes death. BTI products, often sold as granules or dunks, are applied by soaking them in water and using that treated water to thoroughly water the infested plants. Because BTI does not affect eggs, pupae, or adults, multiple applications spaced five to seven days apart are necessary to break the life cycle.
Preventing Recurrence: Watering and Soil Management
Preventing a future infestation depends on changing the environmental conditions that allow fungus gnats to thrive. Fungus gnats require consistently moist conditions near the soil surface to lay eggs and for their larvae to survive. Allowing the top layer of soil to dry out significantly between waterings is the most important cultural change. The goal is to allow the top one to two inches of potting mix to completely dry before watering again.
This “dry-out” method eliminates the moist breeding ground preferred by female gnats. An alternative technique is to water plants from the bottom by placing the pot in a saucer of water, allowing the plant to soak up moisture. Bottom watering keeps the soil surface dry while providing necessary hydration to the roots.
Another effective preventative measure involves creating a physical barrier on the soil surface. Applying a half-inch layer of non-organic material, such as coarse sand, fine gravel, or pumice, deters adults from accessing the moist soil to lay their eggs. Using well-draining potting mixes with materials like perlite or vermiculite, and avoiding mixes heavy in uncomposted organic matter, will also help the soil dry faster.