**Fungus Gnats** (family Sciaridae) are small, dark, delicate flies commonly associated with moist soil and decaying organic matter. They are primarily a nuisance when they infest indoor potted plants, greenhouses, and nurseries, where adults can be seen flying erratically or resting on the soil surface. Their larvae, which live in the soil, can be damaging pests to seedlings, cuttings, and young plants by feeding on roots and plant tissue.
Taxonomy and Classification
Fungus Gnats belong to the order Diptera (true flies). They undergo complete metamorphosis. They are closely related to harmless species (like shore flies) but are distinct because of the larvae’s feeding habits. They thrive in overly saturated soil environments where high moisture content encourages the growth of the fungi and algae that the larvae prefer to eat.
Physical Description
Adult Fungus Gnats are about $1/8$ inch long, grayish to black, with long legs and antennae, giving them a mosquito-like appearance. They are weak flyers and tend to hover over the soil.
The **larvae** are the damaging stage: small, translucent or whitish legless maggots, typically $1/4$ inch long, with a distinct, shiny **black head capsule**. They are found in the top $1$ to $2$ inches of the soil and often leave slimy trails on the soil or pots as they move.
Distribution and Habitat
Fungus Gnats are cosmopolitan. Their habitat is any environment with continuous moisture and rich organic soil, which favors fungal growth. This includes indoor plant containers, poorly drained nursery beds, and compost piles. They are commonly introduced to homes via infested potting soil or new plants.
Behavior and Damage
The life cycle is rapid (as short as 3–4 weeks), leading to constantly overlapping generations. The adults lay eggs directly onto the surface of moist soil.
- **Larval Feeding:** Larvae typically feed on fungi, algae, and decaying organic matter in the soil. However, when fungal food is scarce (or populations are extremely high), they will feed on delicate plant tissues, chewing on **root hairs, fine feeder roots, and soft stems**, especially in seedlings and cuttings.
- **Plant Damage:** Root damage causes visible symptoms above ground, including wilting, yellowing, stunting, and poor growth, often mimicking nutrient deficiency or drought stress.
- **Disease Vector:** Larvae can potentially transmit fungal spores (like *Pythium*) that cause damping-off disease in young plants.
Management and Prevention
Management is centered on eliminating the larvae’s habitat (moist soil).
- **Moisture Control (The Best Method):** Allow the top $1$ to $2$ inches of soil to **dry out completely** between waterings. This kills the larvae and prevents adults from laying new eggs.
- **Exclusion/Trapping:** Place **yellow sticky traps** horizontally on the soil surface to capture large numbers of egg-laying adults.
- **Top Layer Barrier:** Apply a 1-inch thick layer of sterile sand, fine gravel, or food-grade **Diatomaceous Earth (DE)** to the soil surface to create a dry barrier that prevents adults from accessing the soil.
- **Biological Control:** Apply **_Bacillus thuringiensis israelensis_ (Bti)**, often sold as “Mosquito Bits,” as a soil drench. This bacterium is highly effective and specific for fly larvae, killing them internally without harming plants or other organisms.
- **Nematodes:** **Beneficial Nematodes** (*Steinernema feltiae*) can be applied as a soil drench to parasitize and kill the larvae.
Conservation and Research
Fungus Gnats are managed as agricultural and ornamental pests. Research focuses on developing improved biological controls, particularly Bti strains and beneficial nematode applications, for use in sensitive indoor environments and commercial greenhouses.