The sudden appearance of small, dark flies hovering around your beloved Fiddle Leaf Fig (Ficus lyrata) signals a common houseplant problem: a fungus gnat infestation. These tiny insects, belonging to the family Sciaridae, are a nuisance but are generally not a threat to mature Fiddle Leaf Figs. The popularity and specific care requirements of this plant, particularly its need for thorough but infrequent watering, often create the moist topsoil environment that fungus gnats seek out for laying eggs. Successfully removing these pests requires a two-pronged strategy that eliminates both the soil-dwelling larvae and the flying adult population.
Treating the Soil to Eradicate Larvae
The most effective approach to control an infestation focuses on the larval stage, which lives just beneath the soil surface. Before applying any treatment, allow the top two inches of the Fiddle Leaf Fig’s soil to dry out completely. This stresses the moisture-dependent larvae and makes them more susceptible to control measures. Once the soil is dry, two effective drench methods can interrupt the gnat life cycle.
One of the most recommended biological controls is the use of products containing Bacillus thuringiensis israelensis (BTI), a naturally occurring soil bacterium. When gnat larvae ingest the BTI spores, the bacteria release protein toxins that specifically target and disrupt the lining of the larval gut, leading to their death within a few days. BTI is considered safe for plants, pets, and humans because its toxins only activate in the alkaline digestive systems of certain insect larvae. To treat the soil, BTI products, often sold as granules or “dunks,” are soaked in water for about 30 minutes to create an infused solution, which is then used to water the plant thoroughly.
An alternative treatment is a soil drench using a diluted hydrogen peroxide solution. Standard household 3% hydrogen peroxide should be mixed with water at a ratio of one part hydrogen peroxide to four parts water. Applying this mixture to the soil kills the gnat larvae on contact through a rapid oxidation reaction. The solution breaks down into water and oxygen, which you may notice as a temporary fizzing action on the soil surface, without harming the plant’s roots. This method is best used once to knock down a heavy infestation, but wait a day after a hydrogen peroxide drench before using BTI, as the peroxide can neutralize the beneficial bacteria.
Capturing and Removing Flying Adults
While targeting the larvae eliminates the next generation, you must also address the flying adults, which are responsible for reproduction. These airborne gnats are attracted to bright colors and fermented scents, which can be leveraged for trapping. Eliminating adults prevents them from laying new clutches of eggs in the potting mix, which would restart the infestation cycle.
Yellow sticky traps provide a simple, non-toxic physical means of control for the adults. Fungus gnats are strongly attracted to the color yellow, often mistaking it for a food source. Placing these small adhesive cards horizontally near the soil line or inserting them directly into the pot captures the adult gnats as they emerge or fly near the plant. Trapping the adults prevents them from mating and laying eggs.
A simple homemade trap using apple cider vinegar (ACV) can also lure and capture the flying adults. Gnats are drawn to the fermented aroma of the vinegar, which mimics decaying organic matter. To create the trap, combine a small amount of ACV with a drop of liquid dish soap in a shallow container. The dish soap breaks the surface tension of the liquid, causing the gnats that land to sink and drown.
Long-Term Moisture Management and Prevention
Addressing the root cause of the infestation—excessive moisture—is the most reliable long-term solution for preventing recurrence. Fungus gnats thrive in wet conditions, and their life cycle depends on moist topsoil. The Fiddle Leaf Fig is particularly susceptible because its roots are prone to rot in waterlogged conditions, even though its leaves tolerate dry periods.
A change in watering technique is necessary to dry out the upper soil layer where the larvae reside. Adopt a “soak and dry” approach, watering only once the top two inches of soil feel completely dry to the touch, instead of using a fixed schedule. When watering, soak the entire root ball until water flows freely from the drainage holes, then discard any standing water from the saucer immediately. This practice ensures deep hydration while allowing the top layer of soil to dry quickly.
Improving the soil’s composition and drainage is another helpful preventative measure. Using a potting mix that includes amendments like perlite or orchid bark can increase aeration and speed up the drying time of the soil. Additionally, a physical barrier can be placed on the soil surface to discourage adult gnats from laying eggs. A half-inch layer of decorative sand or diatomaceous earth creates a dry, inhospitable barrier that the adults cannot easily penetrate. Always inspect any new plants before bringing them into your home, as they are a common source for introducing fungus gnat eggs and larvae.