Fungus gnats are garden pests that affect the health of vegetable plants. Inexperienced gardeners often regard these small swarming insects as bothersome pests, but the fungus gnat can quickly affect the appearance and viability of a vegetable plant. Plants that are infested but not treated will die from diseases of the plant and/or roots.
The fungus gnat is shiny and black, with thin antennae. According to EdibleContainerGardening.com, adult fungus gnats resemble mosquitoes in shape. They are approximately 3 to 4mm (1/10 to 1/8 inches) long, from head to tail end. Fungus gnats' wings are not strong enough to support high or lengthy flights, so they are most often found flying under the plants' first layer of leaves, or resting on top of the soil.
Female fungus gnats live for approximately 10 days, during which each lays approximately 150 eggs. The eggs hatch in a week or less, and the white larvae have dark black heads. The larvae enter the pupa state in a week or two, and emerge as adults in less than a week. The temperature of the area affects the length of the fungus gnat life cycle. According to the Ontario Ministry of Agriculture, the fungus gnat life cycle, can be completed in 21 days at 75 degrees F (24 degrees C), compared with 38 days at 61 degrees F (16 degrees C).
If you see adult flies on the surface of the soil, take a sample to check for larvae beneath the surface of the soil. If you have an infestation, it is imperative to treat the plant and kill the fungus gnats as soon as possible.
Fungus gnat larvae feed on wet, tender roots. This weakens the plant and leaves it vulnerable to fungal infection. Adult fungus gnats spread fungal infections to a vegetable plant's lower leaves and stems, through infective secretions on their lower legs. If not caught early, these diseases cause the death of the plant.
According to EdibleContainerGardening.com, treatment of fungus gnats on vegetable plants is best done with an environmentally friendly insecticidal soap. Insecticidal soap coats the wings and bodies of adult gnats, eggs, making flight and respiration impossible. Retreatment is often needed.
Fungus gnats require wet environments in which to live and reproduce. To avoid an infestation, keep your soil moist, but never overly wet. If the vegetable plants are planted in pots or containers, drainage holes are extremely important, so that the plant's roots are not sitting in wet soil. A well-drained potting mix (as opposed to water-retaining mixes or topsoil) enables excess water to drain away from the plant's root system quickly.