Succulents are popular houseplants, often chosen for their drought tolerance and low maintenance. When tiny, dark insects hover around the pots, owners often assume the plants are attracting common household flies. Healthy succulents do not attract flies; instead, the soil environment created by certain care practices attracts a specific type of pest. Addressing this issue requires understanding the insect and modifying the conditions that allow it to thrive.
Identifying the Specific Pests
The small, dark insects frequently mistaken for common flies are actually fungus gnats (sciarid flies). These pests are weak, erratic flyers, typically measuring 1/16 to 1/8 of an inch long, with translucent wings and a delicate, mosquito-like appearance. While the adult gnats are mainly a nuisance, they signal a significant problem occurring beneath the soil surface.
The true source of the infestation and plant damage is the larval stage, which resides in the top few inches of the potting medium. These larvae are tiny, translucent, legless worms with distinctive shiny black heads. They primarily feed on decaying organic matter, fungi, and algae within the soil. However, a large infestation can also chew on fine succulent roots. The complete life cycle, from egg to adult, is short, taking about 17 to 21 days, which allows populations to multiply rapidly.
Fungus gnats can be confused with other small, flying pests, but their behavior is distinct. Fruit flies are attracted to fermenting sugars from ripe or rotting produce and are rarely found near the soil. Shore flies are slightly more robust and associated with algae growth, but their larvae feed almost exclusively on algae and organic sludge, not plant roots. Recognizing the fungus gnat is the first step toward effective control, as the solution must target the soil-dwelling larvae.
Why Succulents Create a Suitable Habitat
Fungus gnats are not drawn to the succulent plant itself, but to the persistently moist, organic-rich environment of the potting mix. Their larvae require damp conditions to survive and feed on the fungi and decomposing matter that flourishes there. The presence of these gnats is a direct indicator of overwatering or poor drainage, which creates an ideal breeding ground.
Standard commercial potting mixes often contain a high percentage of peat moss or coir, which are excellent at retaining moisture. When used for succulents, which prefer a fast-draining, arid environment, these mixes stay wet too long. This excessive moisture promotes the growth of fungi the gnat larvae consume, providing them with a continuous food source and a protected nursery for their eggs.
Succulent roots are designed to store water and are highly susceptible to rot in saturated soil. Keeping the soil constantly moist not only risks root rot but also encourages the rapid proliferation of gnat larvae. By maintaining wet soil, the grower disrupts the plant’s natural need for a complete “soak and dry” cycle, creating the wet substrate necessary for the gnat life cycle to continue.
Eliminating the Problem and Preventing Recurrence
Eliminating a gnat infestation requires a two-pronged approach: killing the existing larvae in the soil and capturing the flying adults to break the reproductive cycle. For immediate elimination of larvae, the most effective method is allowing the soil to dry out completely, which desiccates the vulnerable young. If the plant can tolerate it, withholding water until the soil is bone dry for several days will significantly reduce the larval population.
For targeted biological control, the application of Bacillus thuringiensis subspecies israelensis (Bti) is highly effective. This naturally occurring soil bacterium, often sold as “mosquito bits” or “dunks,” is mixed with water and applied as a soil drench. Bti targets and kills only the gnat larvae upon ingestion, posing no threat to the plant, pets, or humans. Alternatively, beneficial nematodes, specifically Steinernema feltiae, can be introduced to the soil, where they actively hunt and parasitize the larvae.
To manage the adult population, yellow sticky traps should be placed near the plants. The bright color attracts the weak-flying adults, trapping them on the adhesive surface and preventing reproduction. This is a simple, non-toxic way to monitor the severity of the infestation and reduce the number of new eggs being laid.
Long-term prevention requires changing cultural practices to eliminate the moist habitat. The primary change is modifying the watering schedule, adopting a “soak and dry” method where the soil dries out entirely between thorough waterings. Repotting the succulent into a gritty, fast-draining mix is also important. A suitable blend should contain a high percentage of inorganic materials mixed with a smaller portion of organic potting soil to ensure rapid drainage and quick drying.