Fungus gnats are small, dark, fly-like insects commonly seen hovering around indoor plants. These pests thrive in moist potting soil, where they lay eggs and rapidly increase their population. While the adult gnats are primarily annoying, the larvae that hatch in the soil can damage delicate plant roots by feeding on them. Eliminating these pests requires a two-pronged approach: capturing the flying adults and treating the soil to destroy the larvae and stop the breeding cycle.
Trapping Adult Fungus Gnats
Targeting the adult fungus gnat population is the first step, as removing these flyers prevents them from reproducing and laying new eggs. One effective, non-toxic method involves yellow sticky traps. Fungus gnats are naturally attracted to the color yellow. These small adhesive cards should be placed directly into the soil near the plant’s surface, where the adults are most active.
A simple technique for catching adults is a homemade liquid bait trap using apple cider vinegar or red wine. The sweet, fermenting scent acts as an irresistible attractant for the gnats. To create a trap, fill a shallow dish with the liquid and add a few drops of dish soap. The dish soap breaks the surface tension, causing any gnat that lands on the surface to sink and drown. For maximum effectiveness, these liquid traps should be changed every few days to keep the scent strong, and they can be covered with plastic wrap with small holes poked in.
Trapping adult gnats immediately reduces visible pests, but it is only a partial solution. Adults represent the final stage of the life cycle, and trapping them does not address the eggs and larvae developing beneath the soil surface. Failure to eradicate the larvae means the cycle will continue, leading to a fresh wave of flying gnats.
Eliminating Larvae in the Soil
The most effective way to end an infestation is by treating the soil to destroy the larvae, which are responsible for reproduction and root damage. One popular biological treatment uses the naturally occurring bacterium Bacillus thuringiensis israelensis (BTI). BTI is a biological larvicide that produces protein crystals toxic specifically to the larvae of certain flies, including fungus gnats and mosquitoes. It is harmless to humans, pets, and plants.
BTI treatment is typically applied as a soil drench, often using commercial products like Mosquito Bits or Dunks. The product is soaked in water for at least 30 minutes to release the BTI spores, and this infused water is used to water the infested plants. When the larvae ingest the BTI, the alkaline conditions in their gut activate the protein toxins. This disrupts the gut lining, causing them to stop feeding and die within 24 to 48 hours.
Another biological option involves introducing beneficial nematodes, specifically Steinernema feltiae (SF). These microscopic roundworms actively seek out and penetrate the fungus gnat larvae through body openings. Once inside, the nematodes release a symbiotic bacterium that multiplies and kills the host larva within a few days. SF nematodes are applied as a soil drench and are most effective when the soil temperature is between 57°F and 79°F. The soil must be kept consistently moist for several days after application.
For a non-biological, readily available treatment, a hydrogen peroxide soil drench kills larvae on contact. A common dilution mixes one part of 3% hydrogen peroxide with four parts water. When poured onto the soil, the peroxide reacts with organic matter, releasing oxygen that rapidly kills the soft-bodied larvae and eggs. This method provides a quick kill, but repeated use may affect beneficial soil microorganisms.
Long-Term Source Control and Prevention
The primary factor attracting fungus gnats is overly moist potting soil, which provides the ideal environment for egg-laying and larval development. Adjusting watering habits is the most significant long-term prevention strategy. Plants should be watered deeply but infrequently, allowing the top one to two inches of soil to dry out completely between waterings.
A physical barrier placed on the soil surface can stop the gnat life cycle by preventing adults from laying eggs and preventing emerging adults from exiting. A half-inch layer of horticultural sand or fine gravel creates a dry, inhospitable top layer. Washed play sand works well for indoor plants, but unwashed sand can compact over time, potentially leading to water absorption issues.
Diatomaceous earth (DE) can also be used as a surface barrier. This fine, abrasive powder desiccates the adults and larvae that cross it. The DE must be food-grade and kept dry to remain effective, as it loses its abrasive quality when wet. Selecting a potting mix that drains well and avoids excessive moisture retention is also helpful, since mixes high in peat moss or certain wood chips hold water for longer periods.