Fungus Gnats

Fungus gnats are small, mosquito‑like flies commonly associated with moist growing media. While the adults are mostly a nuisance, the larval stage is the primary concern. Fungus gnat larvae live in the upper soil profile, often near the surface or around drain holes, where they actively feed on fungi, decaying organic material, and live plant tissue, including fine roots. In propagation environments, larvae may also hollow out the stems of cuttings, disrupting root development and establishment. Fungus gnats thrive in wet, over‑watered conditions and are strongly associated with poor drainage. Beyond direct feeding damage, both larvae and adults are known vectors of plant diseases, a risk that is often underestimated. Larvae can transmit root pathogens such as Pythium, Fusarium oxysporum, Verticillium albo‑atrum, and Thielaviopsis basicola (black root rot) as they move through the growing media. Adult fungus gnats can also spread foliar diseases, including Botrytis, by carrying spores on their bodies. While fungus gnat adults and larvae are generally straightforward to control, sound irrigation practices and proper drainage are critical for long‑term prevention. When fungus gnats persist or return repeatedly, cultural conditions, especially excess moisture, are often the underlying cause.

Common signs of fungus gnats include slowed or stunted plant growth, adult gnats hovering near the soil surface, and larvae present in the growing media. Small gnats flying up when a plant is disturbed or being caught on sticky traps are also strong indicators.

The fungus gnat life cycle has 4 stages:Eggs are laid by adult females into the cracks and crevice of most soil, not directly on the soil surface. There are 4 larval stages, characterized by a black ‘head capsule’. The larvae feed on root hairs, the stems of unrooted cuttings, and organic material. They may be located near the top of the container, or the bottom. The pupal stage does not feed. Adult fungus gnats emerge after pupation, and are poor flyers. The life cycle takes between approximately 18 to 28 days, depending on temperature. Adults highly prefer wet soils to dry. Females can lay between 75 and 200 eggs in her lifetime. Adults are can easily be distinguished by their bead-like antennae, the the ‘Y’ shaped vein in their wings. Yellow sticky cards, or potato slices monitored every 48 hours, can be used to assess adult or larvae populations, respectively.

There are a few options for control of fungus gnats. Some growers prefer the use of Nematodes such as Stenernema feltiae, while others choose to use Stratiolaelaps scimitis. Both of these pair well with Dalotia coriaria to gain the upper hand on fungus gnats. Apply heavier amounts up front for 2-3 weeks in a row has been the most effective to overpower a cycling fungus gnat populations.

The same predators used to clean up a fungus gnat population can work very well as preventatives. The rate of application would be less, and you would only need to apply every 2-4 weeks to keep the population of beneficials at high levels.