Fungus gnats are tiny, dark insects (Sciaridae family) that commonly infest houseplants, thriving in moist soil environments. Adult fungus gnats do not eat plant leaves and are mostly harmless flyers. The damage is caused by their soil-dwelling larvae, which primarily feed on organic matter and fungi. However, larvae will consume plant roots when their preferred food source is unavailable. Understanding these two distinct life stages is the first step toward effective management of an infestation.
What Adult and Larval Fungus Gnats Actually Consume
Adult fungus gnats are mainly a nuisance. These weak fliers hover around plants or run across the soil surface. They have a short lifespan and feed minimally, typically consuming only liquids like water or flower nectar. The adult’s main purpose is to mate and lay eggs in the moist top layers of the soil.
The larval stage is the destructive phase. Larvae are worm-like and translucent with a distinct black head. They primarily sustain themselves by consuming fungi, algae, and decaying organic matter in the potting mix. This preference explains why gnats are attracted to consistently wet soil, as moisture encourages the growth of their food sources. When population density is high or preferred food is depleted, the larvae will turn their attention to plant tissue.
Why They Are Harmful to Plants
The harm to plants stems from the larvae feeding directly on delicate root structures beneath the soil surface. Larvae specifically target fine feeder roots and root hairs, which are the structures responsible for absorbing water and nutrients. When these roots are damaged, the plant loses its ability to hydrate and feed itself properly, which results in visible decline.
Physical root damage often manifests as symptoms mimicking other problems, such as yellowing foliage, stunted growth, and a sudden wilting of the plant canopy. The damage is particularly severe for young seedlings and cuttings, which have a less developed root system to withstand the feeding pressure. Heavy larval feeding in these vulnerable plants can quickly lead to death.
A significant secondary harm is the transmission of soil-borne diseases. As larvae move through the soil and feed on roots, they create open wounds that serve as entry points for pathogens. Both adult and larval gnats can carry fungal spores on their bodies, including those that cause destructive diseases like Pythium, Fusarium, and Botrytis. The combination of physical injury and pathogen introduction makes infestations potentially deadly.
Strategies for Eliminating Infestations
Controlling fungus gnats requires targeting both flying adults and soil-dwelling larvae. The most effective environmental control involves managing soil moisture, since larvae cannot survive in dry conditions. Allowing the top two to three inches of potting mix to dry out completely between waterings eliminates the moist environment needed for eggs to hatch and larvae to thrive.
Yellow sticky traps placed horizontally near the soil surface capture the weak-flying adults and monitor population size. Reducing the adult population helps break the life cycle, as adults cannot reproduce without laying eggs. However, physical traps alone will not eliminate existing larvae in the soil.
For direct treatment of the larvae, biological control agents are the preferred method. Bacillus thuringiensis israelensis (Bti) is a naturally occurring bacterium applied as a soil drench. When larvae ingest Bti, it releases proteins that disrupt their digestive system, leading to death without harming plants or pets.
Other highly effective biological solutions involve applying beneficial organisms to the soil. These organisms actively hunt and consume the fungus gnat larvae, providing long-term control.
Other Biological Solutions
Entomopathogenic nematodes, specifically Steinernema feltiae.
Predatory mites, such as Stratiolaelaps scimitus.