I recently repotted my houseplants and have been battling a tiny fly infestation ever since. I’ve tried exterminator services, soapy water, dryer sheets — you name it — the flies are still thriving. Do you have any guidance for me?
— Cindy Golan, Park Ridge
Gardening is supposed to be a relaxing hobby, but like you, I sometimes find working with plants rather frustrating. Your description sounds like you have an ongoing healthy infestation of fungus gnats. They are a common pest of plants grown indoors and are often associated with overwatered houseplants. They can become a nuisance when they are present in large numbers and fly around inside a home. The adult fungus gnat is a small (⅛-inch-long), dark, mosquito-like insect with long legs and one pair of clear wings. It’s harmless (it does not bite) but can be very annoying in large numbers and generally remains at the base of the plants. They are usually first noticed flying around houseplants as the plants are watered or moved. You may also see them gathered near any adjacent windows.
Adult fungus gnats live for seven to ten days. During this time, females can lay 200 to 300 eggs in the potting soil. The larvae emerge in four to six days and feed on the plant’s roots and any decaying plant material. Found in the top two inches of soil, larvae have a shiny black head and a ¼-inch-long, elongated whitish to transparent body. You can tell if the larvae are present by placing a few very thin slices of raw potato on top of the soil. The larvae will begin to feed on the potato within a few days. Look at the underside of the potato slice too. This stage of the insect can cause damage to plants. The pupal stage will last three to four days before the young adults emerge and begin the next generation. The entire life cycle from egg to adult may be completed in as little as three weeks, depending on room temperature, so potted plants can host each life cycle stage of egg, larvae, pupae and adult in multiple generations at the same time.
Houseplants can show sudden wilting, loss of vigor, poor growth and yellowing when heavily infested with fungus gnats. When the infestations are heavy, a considerable portion of the plants may be killed, so taking measures to control them is important. Since this insect prefers moist conditions, allow the growing medium to dry to a depth of one to two inches before watering again. This helps kill the larvae and inhibit the development of eggs while making the soil less attractive to egg-laying females. If possible, water the plant from the bottom, but empty the saucer after an hour. Once you have the pest eliminated, be careful not to overwater in the future.
Other control options include installing yellow sticky cards, available at garden centers. Attach the cards to wooden skewers or sticks and place horizontally at the pot’s surface to attract and capture the fungus gnats. Replace the sticky cards as needed; it may take time to get them under control using this method. Another option is to mix one part 9% hydrogen peroxide with six parts of water and drench the top layer of growing medium in the pot once it has completely dried out.
There are commercially available biological control agents that can be purchased to control fungus gnats in pots. Several Bti products (the biological insecticide Bacillus thuringiensis subspecies israelensis) are sold in retail nurseries and garden centers. Bti does not reproduce or persist indoors, so fungus gnat infestations in potting media might require repeated applications at about five-day intervals. You can also try spraying the top of the growing medium lightly with an insecticidal soap when you spot the pests.
Modifying your watering practices will be key to controlling fungus gnats and preventing their reappearance. I have not had issues with fungus gnats and manage my houseplants on the dry side — generally too dry as they often wilt, which is not good practice.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.
