The appearance of small, dark flies buzzing around potted succulents often indicates an infestation of fungus gnats. These pests are attracted to moist soil and decaying organic matter, which provides the perfect environment for them to lay their eggs. Adult fungus gnats are mostly a nuisance, but their larvae live in the top layer of soil and can feed on the delicate root hairs of succulents, causing damage, especially in seedlings or already stressed plants. The entire life cycle, from egg to adult, can complete in as little as three to four weeks, meaning a small problem can quickly become a large one if not addressed.
Adjusting Watering Practices
The most effective step in eliminating a fungus gnat problem on succulents is to change your watering routine. Fungus gnats require consistently moist conditions for their eggs and larvae to survive, so a wet top layer of soil is ideal for reproduction. Succulents, being desert plants, already thrive on a “soak and dry” method, where the soil is allowed to dry out completely between waterings.
This practice is the direct opposite of what the gnats need to reproduce and will cause existing eggs or larvae in the upper soil layer to dry out and die. Check the soil moisture deep down, perhaps an inch or two below the surface, to ensure it is completely dry before watering. A change to bottom watering can also be highly beneficial during this time, as it allows the plant to absorb water from the drainage holes while keeping the top inch of soil dry. This dry surface layer prevents adult gnats from laying new eggs and breaks the reproductive cycle.
Trapping and Removing Adult Gnats
You must eliminate the current population of flying adults to stop them from laying more eggs, as adult fungus gnats can lay up to 200 eggs in their short seven to ten-day lifespan. The use of yellow sticky traps exploits the gnats’ natural attraction to the color yellow.
These small, adhesive cards should be placed horizontally at the soil surface or vertically on a stake near the plant. A simple homemade trap can also be deployed by mixing a shallow dish of apple cider vinegar or wine with a few drops of dish soap. The soap breaks the liquid’s surface tension, causing the gnats, which are attracted to the fermenting smell, to fall in and drown.
Targeting Larvae in the Soil
Eradicating the larvae residing in the top two inches of the potting mix is necessary, as they are the source of the infestation and the stage that can damage roots. One effective biological control involves using products containing Bacillus thuringiensis subspecies israelensis (Bti), a naturally occurring bacterium.
Bti is mixed with water and applied as a soil drench, where the gnat larvae ingest the bacteria’s protein toxins, leading to their death within 24 to 48 hours. This biological agent is highly selective, meaning it is harmless to plants, pets, and humans. The Bti treatment should be repeated with every watering for several weeks to ensure all newly hatched larvae are eliminated.
A physical method involves applying a layer of fine, dry material to the soil surface, like diatomaceous earth (DE) or coarse sand. Diatomaceous earth is a fine powder that works by physically slicing the exoskeletons of the larvae and adult gnats that crawl through it, leading to dehydration. For safety, only food-grade DE should be used, and it must be kept dry to remain effective.
An alternative chemical treatment is a diluted hydrogen peroxide drench. This solution is created by mixing one part of standard 3% hydrogen peroxide with four parts of water. When poured onto the soil, the peroxide kills the larvae on contact and sterilizes the soil by destroying fungi. This method should not be used immediately after a Bti application, as the peroxide will kill the beneficial bacteria.
Maintaining a Gnat-Free Environment
Once the immediate infestation is under control, preventative measures are necessary to keep the environment inhospitable to fungus gnats. Proper drainage is important; succulents should always be planted in containers with drainage holes, using a well-draining succulent or cactus mix to prevent the soil from becoming overly moisture-retentive. You should also be diligent about removing any dead leaves or debris from the soil surface, as this organic matter serves as a food source for the gnat larvae. Sterilize the soil before repotting to eliminate any potential eggs or larvae, and control the general humidity in the area to discourage the moist conditions gnats require.