Plant flies found around houseplants are most often fungus gnats, which are small, dark insects. While the flying adults are primarily a nuisance, their presence indicates a problem in the soil below. These weak fliers lay eggs in moist potting mix, signaling the soil is likely remaining too wet. The larvae that hatch are the true threat, feeding on decaying organic matter, fungi, and, in severe cases, the fine root hairs of the plant. A large population of these larvae can stunt the growth of young plants and seedlings.
Quick Fixes for Adult Flies
Addressing the flying adult gnats immediately reduces the number of females laying eggs. Yellow sticky traps are a highly effective, non-toxic physical solution for capturing these pests. The bright yellow color attracts the gnats, which become stuck to the adhesive surface. Placing these traps horizontally on the soil or inserting them vertically helps continuously remove the reproductive population.
Another simple method involves creating a household liquid trap. A shallow dish filled with apple cider vinegar, a few drops of dish soap, and water will attract the adults. The soap reduces the surface tension, causing the gnats that land to sink and drown. For localized swarms, a handheld vacuum cleaner can provide instant control by quickly sucking up the weak-flying adults. While these methods offer quick results by catching adults, they do not eliminate the existing larvae in the soil, which is the source of the problem.
Eliminating the Source in the Soil
The solution requires treating the potting mix to kill the larvae and eggs, as the larval stage is responsible for the damage. A primary biological control is a soil drench using Bacillus thuringiensis israelensis (BTI). BTI is a microbial insecticide that must be ingested by the larvae, releasing a toxin that paralyzes their digestive system. This treatment is harmless to plants, pets, and beneficial insects outside of the fly family.
To apply BTI, granules are steeped in water to create a concentrated “tea,” which is then used to water the affected plants. This application must be repeated weekly for at least three weeks to eliminate new larvae hatching from eggs. Another effective drench involves using diluted household hydrogen peroxide, mixed at a ratio of one part 3% peroxide to four parts water. When poured over the soil, this solution rapidly oxidizes, creating a temporary foaming action that kills the larvae on contact.
For a physical barrier treatment, a layer of Diatomaceous Earth (DE) can be applied to the soil surface. This fine powder consists of fossilized remains of diatoms. The DE works by lacerating the exoskeleton of crawling larvae and absorbing moisture from emerging adults, causing them to dehydrate. The layer should be about a quarter-inch thick and must remain dry to maintain its effectiveness.
Preventing Future Infestations
Preventing fungus gnats involves altering the plant’s environment to make it inhospitable. Since gnats require consistently moist soil to breed, changing watering habits is the primary adjustment. Allowing the top one to two inches of potting mix to dry out completely between waterings dries out the top layer where females lay eggs and larvae reside.
Another technique to prevent surface moisture is bottom watering. By placing the pot in a saucer of water, roots absorb moisture from below, keeping the upper soil layer dry and less attractive for egg-laying. Attention to the potting medium also helps prevent recurrence, as gnats are attracted to decaying organic matter. Using sterile, fresh potting mix and avoiding unpasteurized compost reduces the initial food source for the larvae.
Creating a physical barrier on the soil surface is an effective preventative measure. A layer of coarse sand, decorative gravel, or fine horticultural pebbles applied over the topsoil creates a dry zone. Females cannot easily penetrate this layer to lay their eggs. This top dressing seals off the moist sub-surface, effectively breaking the life cycle and preventing new infestations.