Pesticides and bacterial control agents, such as nematodes, are used to treat fungus gnat infestations. Eliminating the conditions that attract gnats helps to control the infestation.
Fungus gnats can be controlled indoors by using yellow sticky traps in the affected areas. The sticky traps capture adult gnats but do not kill fungus gnat larvae. To eliminate larvae, it is necessary to treat the affected soil with an appropriate pesticide or biological control agent. To treat plants with pesticide, apply the pesticide to the top 1 inch of soil. Pyrethroids and encapsulated pesticides, such as chlorpyrifos, effectively kill adult fungus gnats and larvae when applied to the infested soil.
Certain species of nematodes, mites and bacteria are used as biological control agents to eliminate gnats in greenhouses and gardens. Apply the biological control agent according to the directions on the product packaging, and continue to monitor the presence of gnats using yellow sticky cards or potato chunks placed on top of the affected soil. Examine the cards and potato chunks regularly to determine how often to treat the soil. Some biological control agents must be applied several times before gnat populations are well-controlled.
Fungus gnats are attracted to damp soil and incompletely composted organic matter. Do not overwater plants, and avoid using manure and other incompletely composted materials in gardens to prevent fungus gnats. If possible, move plants with severe infestations away from other plants.