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Do you have a swarm of small black flies rising from your flower pot? If the answer is yes, then these little flies called fungus gnats have nested. They are not fruit flies or mosquitoes.
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What Are Fungus Gnats?
Fungus gnats, also known as sciarid flies, are tiny black flies, usually common pests in indoor plants (home, nurseries and greenhouse crops). The larvae of fungus gnats (Bradysia spp., Lycoriella spp.) are often found in the potting soil of newly purchased plants from garden centers or hardware stores. They are dipterans belonging to the Sciaridae family. Adult fungus gnats are about 3–4 mm long and usually occur in swarms. They are most noticeable when you water as a small swarm might fly toward you and around plants. They are easily recognized by two long transparent wings, long legs and antennae. Their larvae are about 6 mm long when mature, translucent with a shiny black head and legless. Bradysia spp. are commonly found in greenhouses, whereas Lycoriella spp. are common in mushroom crops.
Fungus Gnat Life Cycle
The life cycle of a fungus gnat is made of four stages —egg, four larval instars, pupa, and adult, and takes about 20–28 days at 20–25 °C. The eggs develop into larvae in three to four weeks, depending on the temperature. Due to overlapping generations, fungus gnats can be active all year round. During their short lifetime, females lay between 100–200 eggs in moist soil. Eggs hatch in four to seven days, producing white larvae that can reach up to 4–6 mm long. The larvae feed for five to fourteen days before pupating, after which adults emerge in four to six days. Fungus gnats feel particularly comfortable in nutrient-rich, moist soil.
Are Fungus Gnats Harmful?
Fungus gnats are harmless to humans, but can quickly become annoying when they emerge and fly around within the home environment in a swarm. Although fungus gnats resemble small mosquitoes, they cannot bite. Even though larvae are rarely seen on the pot surface, they prefer to feed on young plants and can damage plant seedlings, especially when growing tomato or other vegetable plants with tender seedlings. Damaged areas of plants can then be an entry point for pathogens. Wilting, yellow leaves, poor growth, and dying plants can be indicators of a fungus gnat infestation as fungus gnat larvae bore into the roots and up the stems of seedlings, cuttings and young plants. The plants can no longer absorb sufficient water and nutrients if the roots are damaged.
Control of Fungus Gnats With Beneficial Nematodes
Fungus gnats can be controlled by using natural enemies such as beneficial nematodes. For example, Steinernema feltiae nematodes are effective against the larval stages of fungus gnats that live in the soil. Nematodes are naturally occurring soil organisms. They are very small (0.3–0.7 mm) and therefore invisible to the naked eye. They are harmless to humans, animals and plants. Nematodes, often in formulations, are mostly dissolved in a little water, stirred and then added to the irrigation water for plants. After just a few weeks, the fungus gnats will have disappeared.
Learn more about how to use beneficial nematodes to combat insect pests in your garden here.