Fungus gnats are a common houseplant nuisance, identifiable as small, dark, mosquito-like flies that hover around the soil of potted plants. While the adult flies are mostly just annoying, the true damage is caused by their translucent, black-headed larvae living in the top few inches of the growing medium. These larvae feed on decaying organic matter and, on delicate plant roots, especially in seedlings and young plants, leading to wilting and stunted growth. To combat this pest effectively, many people turn to entomopathogenic nematodes (EPNs), which are microscopic, living organisms used as a biological control agent. EPNs are naturally occurring, beneficial roundworms that actively hunt insect pests in the soil without posing a risk to plants, people, or household pets.
Selecting the Proper Nematode Strain
The most effective strain for biological control against fungus gnats is Steinernema feltiae (S. feltiae), which is widely available under various trade names. This particular nematode is highly valued because its foraging strategy is perfectly suited for the environment where fungus gnat larvae reside. S. feltiae is classified as a “cruiser” type, meaning it actively moves through the soil profile to seek out its prey rather than waiting in ambush.
This cruising behavior allows S. feltiae to efficiently locate the fungus gnat larvae and pupae within the top two to three inches of the growing medium. Once a nematode finds a host, it enters through a natural body opening and releases symbiotic bacteria, which multiply and kill the larva within 48 to 72 hours. Other nematode species are generally ineffective against fungus gnats, as they may target different pests or prefer deeper soil layers. For instance, nematodes primarily sold for controlling lawn grubs will not provide satisfactory results against fungus gnat larvae.
Preparation and Handling of Nematodes
Proper handling of nematodes is essential to ensure their effectiveness. Upon arrival, the package, often in a sponge or granular carrier, should be stored immediately in a refrigerator at a cool temperature, typically around 41°F (5°C). Do not freeze them, as freezing will kill the organisms; check the expiration date since they are highly perishable. On the day of application, allow the nematodes to warm up gradually to room temperature before mixing.
Create a concentrated stock solution to allow the nematodes to hydrate and disperse evenly. Mix the entire contents of the package into a smaller volume of cool, clean water in a bucket. The water temperature must be moderate, ideally between 60°F and 75°F (15°C and 24°C), and it should be non-chlorinated, as chlorine can be toxic to the nematodes. If using tap water, letting it sit out overnight allows the chlorine to dissipate naturally, making it safe for the application. Gently stir the mixture for at least five minutes to fully activate the nematodes before diluting for final application.
Step-by-Step Application Techniques
The growing medium must be moist, but not heavily saturated, before treatment, as moisture is necessary for the nematodes to move and hunt efficiently. Apply the solution during the evening, at dusk, or on a cloudy day, because S. feltiae is highly sensitive to ultraviolet light and can be killed by direct sunlight. Applying the solution at cooler times protects the organisms and maximizes their survival rate.
The nematode solution can be applied using a standard watering can or a pump-style sprayer. If a sprayer is used, all fine filters and screens, particularly those finer than 50 mesh, must be removed to prevent the nematodes from becoming clogged or damaged. The solution must be continuously stirred or agitated during application to prevent the nematodes from settling at the bottom of the container, which would result in uneven treatment.
Apply the solution as a soil drench, ensuring the entire surface of the pot is uniformly saturated. The target zone is the top layer of the growing medium, where the fungus gnat larvae feed. Immediately following the drench application, a light watering with plain, non-chlorinated water helps wash any nematodes off the plant foliage and into the soil. Maintain consistent soil moisture for the next seven to ten days, as the nematodes rely on the water film in the soil to move and actively seek out their prey.
Evaluating Success and Follow-up Treatments
Once the nematodes have been applied, the user should manage expectations regarding the timeline for seeing results. The reduction in the pest population is not instantaneous because the treatment specifically targets the larval stage, not the adult flies. Maximum control of the larval population typically occurs over a period of two to four weeks following the initial application. The primary indicator of success is a noticeable decrease in the number of adult fungus gnats flying around the plants, as the treatment breaks the life cycle by eliminating the next generation.
Because nematodes do not affect the eggs laid by adult gnats, a second application is often necessary to achieve complete eradication after the existing adults die off naturally. A follow-up treatment 10 to 14 days after the first application is generally recommended to target any newly hatched larvae that were still in the egg stage during the initial drench. This second application ensures the entire life cycle is interrupted, preventing the next wave of adult flies from emerging.