Last year, I had tiny black flying insects living in the pots on my patio. They didn’t seem to harm the plants, but I was afraid to bring them in for the winter. How can I prevent these pests?
Jo Ann Courtney, Lexington, KY
Leanne Pundt, greenhouse IPM coordinator for the University of Connecticut Cooperative Extension Service, responds: It sounds like you have fungus gnats. Adult fungus gnats are 1/8-inch-long, slender, flying insects with long legs and antennae. They lay their eggs in the upper layer of any moist potting soil that is high in organic matter. The eggs hatch into small, white, legless larvae with black, shiny heads. Fungus gnat larvae feed upon fungi and decaying organic material during their two-week lifetime.
The larvae may also feed on young, tender plant roots. Young seedlings and cuttings are more vulnerable to feeding damage than mature plants are. Inspect the root systems for signs of damage. Roots that have been fed on by fungus gnat larvae tend to have blunt tips.
Keeping your plants on the dry side is the best way to reduce fungus gnats. Adults will be less likely to lay eggs in potting soil if the upper layer is dry. Fungus gnats, like many flying insects, are attracted to yellow, so you can use yellow sticky traps, available through garden supply catalogs, to trap the adults. Yellow plastic picnic plates coated with petroleum jelly also work well.
If these methods fail and your plants show signs of root damage and wilting, or you see larvae in the pots, you may need to treat the soil with Bacillus thuringiensis (Bt). For fungus gnat larvae, you need to look for Bt ssp. israelensis or serotype H-14. It is sold under the trade name of Knock-Out Gnats or Gnatrol. Apply it as a drench, outdoors, to sufficiently wet the soil surface where the larvae are found. Repeat applications may be needed. The fungus gnat larvae ingest the bacterium, stop feeding, and die in a few days.