Both the larval and adult stages of this small plant pest can cause big problems for houseplants. Here’s how to get rid of fungus gnats indoors.
Fungus gnats (families Mycetophilidae and Sciaridae) are a common pest of plants grown indoors, especially where humidity and moisture are high. They’re usually first noticed when the harmless adults are seen flying around house plants or gathered at a nearby window. These non-biting adult gnats can become a flying nuisance. But it’s the larval stage, feeding in the soil, that can damage tender plant roots.
Adults are delicate, grayish black, mosquito-like flies (1/8 inch long) with long legs and one pair of clear wings. They are not strong fliers and emerge from potted plants, especially when watering. Larvae or maggots (1/4 inch) have a shiny black head and an elongated, whitish to transparent body. They are most abundant in damp, rich soils and feed on root hairs, fungi and other organic materials.
Adults live about one week and lay up to 300 eggs in rich, moist soils. Within 4-6 days tiny larvae emerge and begin feeding on plant roots during their two week period. The pupal stage lasts 3-4 days before young adults leave the soil and begin the next generation. The entire life cycle from egg to adult may be completed in as little as 3-4 weeks depending on temperature. Because of their proclivity and relative short gestation, potted plants can host each stage — egg, larvae, pupae, adult — in multiple generations at once. Because of this remedies usually require repeated applications until there are no surviving eggs.
Plant symptoms that indicate fungal gnats are seen as sudden wilting, loss of vigor, poor growth, and yellowing. With severe infestations, a considerable portion of the plants may be lost. Especially prone to injury:
- African violets
Note: Larvae are most damaging to seedlings, cuttings and young plants.
Fungus Gnat Control
- Inspect plants thoroughly prior to purchase for signs of insect pests. Turn up soil carefully near the base of the plant and look for the glossy, clear larvae. Reject any plant sending up flying gnats.
- Fungus gnats do best in damp soils; be careful not to overwater, especially during winter months when plants use less. When potting, avoid water holding, organic material such as peat moss that may encourage egg laying.
- If pests are present, allow the soil to dry to a depth of one to two inches between waterings. This not only kills larvae and inhibits the development of eggs, it also makes the soil less attractive to egg-laying females. Apply Gnat Nix as a top dressing to significantly reduce pest populations.
- Use Yellow Sticky Traps placed horizontally at the soil surface to capture large numbers of egg laying adults. The gnats are attracted to yellow and are easily removed on the trap before they can lay more eggs.
- Mosquito Bits and Microbe-Lift® BMC contain Bacillus thuringiensis var. israelensis, a highly selective biological pesticide that kills damaging larvae, yet is SAFE for fish, wildlife, humans and non-target species. Contains NO harmful residues.
- Top dress houseplants with Beneficial Nematodes to destroy the larvae stage. Nematodes are microscopic round worms that penetrate fungus gnat larvae, as well as harmful lawn and garden grubs , fleas, and other soil-borne pests (they do not harm earthworms), then release a bacterium that consumes the pest from the inside out. The long-lasting nematodes are safe for use around pets, plants, and your family.
- AzaMax contains azadirachtin, the key insecticidal ingredient found in neem oil. Mix 1 Tbsp/ gallon of water and apply as a soil drench for effective control of soil-borne insect larvae. With high pest pressure make applications every 5 to 6 days.
- Flying Insect Killer, a combination of peppermint, cinnamon and sesame oils, is a non-toxic spray that will get rid of gnats and other insects that gather around windows.
Tip: The Fungus Gnat Predator (Hypoaspis aculeifer) is a tiny, but effective killer of fungus gnat and other harmful larvae found in soil. On release, they make for a slow but steady, persistent decline in pest numbers. This beneficial insect prowls the soil attacking the larvae and feeding on their contents. Release 10,000 predators per 200 – 1,000 square feet depending on pest levels.
Fungus Gnat Predator
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