Houseplants can attract small flying insects, and the answer to whether they attract flies is generally yes. The attraction is not to the plant’s foliage but to the environment created by the potting soil and moisture. The presence of these insects is typically a sign of an imbalance in the plant’s care routine, specifically concerning water retention in the soil. While mainly a nuisance to homeowners, their presence indicates conditions that can potentially harm the plant’s root system over time. The primary pest responsible is the fungus gnat, which finds the damp, organic material of a potted plant to be an ideal breeding location.
The Primary Culprit: Fungus Gnats
The small, dark insects often seen hovering around houseplants are usually fungus gnats (family Sciaridae). These delicate-bodied flies are about 3 to 4 millimeters in length, giving them a mosquito-like appearance with long, slender legs and antennae. They are weak, erratic fliers that tend to stay low, making short, unsteady flights near the soil surface.
Fungus gnats are distinct from fruit flies, which prefer decaying fruit, or drain flies, which cluster near plumbing. The adult fungus gnat is mostly a nuisance pest and does not bite or sting. The true problem lies in their life cycle, which includes four stages: egg, larva, pupa, and adult.
Adult females lay tiny, translucent eggs in moist soil, which hatch into larvae after about four to six days. The larvae are clear or whitish, elongated, and have a distinct shiny black head capsule, growing up to a quarter-inch long. These larvae are the destructive stage, as they feed on fungi, algae, and organic matter in the soil. In heavy infestations, they will chew on the fine roots of the plant, leading to poor growth and yellowing leaves.
Conditions That Attract Flies
The primary mechanism of attraction for fungus gnats is consistently moist, organic-rich potting medium, which serves as a necessary breeding site and food source for the larvae. The most common trigger for an infestation is overwatering, as the adult female seeks saturated soil to deposit her eggs. Larvae require this damp environment, typically the top inch or two of the soil, to survive and develop.
High humidity and lack of adequate drainage also contribute to this ideal breeding ground by keeping the soil saturated for extended periods. Many common potting mixes contain high levels of organic matter, such as peat moss or bark, which are designed to retain moisture. This composition also provides an abundant food supply of fungi and decaying organic material for the gnat larvae.
A plant’s water needs fluctuate based on environmental conditions like light and temperature. When the soil remains excessively wet, it promotes fungal growth and can lead to root rot. This decaying material further attracts the gnats, as their larvae feed on the rotting roots. The gnats indicate that the conditions in the pot are favorable for them to complete their life cycle.
Eliminating Plant Pests
Eliminating a fungus gnat infestation begins with cultural control techniques aimed at disrupting the life cycle by removing the moist environment the larvae require. The most important step is to allow the top inch or two of the potting mix to dry out completely between waterings. This method starves the larvae and prevents adult females from laying new eggs in the dry soil.
Bottom watering, where the pot absorbs water from a tray below, is a technique that helps keep the topsoil dry while still hydrating the plant’s roots. For immediate reduction of the adult population, yellow sticky traps are highly effective. The bright yellow color attracts the weak-flying adults, trapping them and preventing further reproduction.
Targeted soil treatments are necessary to eliminate the larvae, which are the most damaging stage. A common household solution involves mixing one part of standard 3% hydrogen peroxide with four parts of water. Pour this solution directly onto the soil until it drains out. The hydrogen peroxide reacts with organic matter, quickly killing the soft-bodied larvae on contact before breaking down into water and oxygen.
Another effective treatment is the application of Bacillus thuringiensis subspecies israelensis (BTI), a naturally occurring bacterium. BTI is target-specific; when applied to the soil, it is ingested by the fungus gnat larvae. Once ingested, BTI releases toxins that disrupt the larvae’s digestive system, halting their development and killing them before they mature. BTI can be applied by steeping granules in water and then using that water to irrigate the plants.