Fungus Gnats

Problem type: Insect

Name of problem:
 Fungus gnats

Plant name(s): Flowering and foliage house plants

Symptoms / Characteristics:
Fungus gnat larvae inhabit the soil where they feed on fungi and decaying organic material. When insect populations are high the larvae often begin feeding on plant roots. Excessive root damage can cause sudden wilting, leaf drop, decreased vigor and stunted growth. Stressed plants are particularly susceptible to fungus gnat injury. Healthy, vigorous plants will usually tolerate mild infestations. Fungus gnat outbreaks are most severe in moist soils, especially those containing peat moss. Overwatered house plants provide an ideal habitat and breeding ground for the insects.

Fungus gnats often originate in potting soil prior to planting or they may be introduced when house plants are brought indoors in the fall. Fungus gnats do exist in garden soils but are perhaps more severe in house plants due to the absence of natural predators. The larvae are whitish semi-transparent maggots with black heads. They can reach a mature length of about 0.5 cm. Although the larvae are responsible for the damage, the adults are easier to detect. Adults are small (0.25 cm) greyish flies that may be seen "jumping" out of the soil or hovering around the plants. Eggs are laid on the soil and the larvae hatch in a matter of days. If conditions are favorable, the entire life cycle can be completed in only a few weeks. Without proper treatment, the population can increase exponentially and the effects can be devastating.

Control / Preventions:
At the time of purchase, ensure that there are no fungus gnats inhabiting the soil and select only healthy, vigorous plants. Use only sterile, properly sealed potting mix inside the home in order to prevent the introduction of fungus gnats. House plants that are kept outdoors during the summer should be carefully inspected before bringing them back into the house. It is a good idea to keep these plants isolated from other house plants for a few weeks. If fungus gnats are detected, an appropriate house plant insecticide should be applied promptly to prevent further infestations.
Avoid high-moisture conditions that attract the insects and aid in their development. Use recommended container soil mixes and ensure proper drainage. Avoid overwatering, especially after bringing house plants indoors after the summer. Remember that these house plants do not require as much water as they did outdoors. Allowing the soil to dry slightly between watering will effectively deter the adults and prevent them from laying eggs. Constant moisture only encourages the development of soil fungi, a prime food source for fungus gnat larvae. A 0.5 cm of sand on top of the soil surface will also deter egg-laying adults that are in search of high moisture soils.

Yellow sticky traps can be used to trap adults and monitor fungus gnat populations. Placing a thin slice of potato on top of the soil surface will attract actively feeding larvae, which can then be discarded. Heavy infestations may warrant the use of a houseplant insecticide. Always follow the manufacturer's label carefully