Are fungus gnats the problem with your greenhouse crop’s roots?

Scout now for fungus gnat larvae in greenhouse crops. Control measures include insecticides and beneficials.

Fungus gnat larvae can burrow into the roots and up the stem of cuttings that you’re trying to root. The results are similar to what you see in Photo 1, where all the roots have been damaged and ultimately cause the plants to die. It also has been reported that fungus gnat adults (Photo 2) can carry fungal spores in the greenhouse with Botrytis, Pythium, Fusarium, Phoma and Verticillium being the most mentioned fungi.

Monitoring with yellow sticky cards laid horizontally near the plant pot surface generally collects twice as many adult fungus gnats as a vertically place card on a florist stake. The adults are easily identified with a 10X or 15X hand lens. Fungus gnat adults are dark-bodied, slender, long-legged, mosquito-like insects. The larvae (Photo 3) are wormlike with a black head capsule and a white to transparent body.

Once cuttings are established, allowing the soil to dry between watering is the best way to reduce fungus gnat infestations. Insecticides recommended to reduce the risk of root feeding or stem feeding include the following insecticides applied as soil drenches according to MSU Entomologist Dave Smitley. Products include: AzatinXL, Adept (not on poinsettias), Benefit, Distance, Marathon or other nicotinoids applied as a soil drench.

Biocontrol of fungus gnat’s larvae may be possible using Hypoaspis miles, a predatory mite, and Atheta, a predatory beetle, although test data are not yet available to support this. Predators should be applied when the fungus gnat population is low. Do not apply insecticides when using beneficials like Hypoaspis.

Fungus gnat larvae feeding damage to roots of phlox cuttings.
Photo 1. Fungus gnat larvae feeding damage to roots of phlox
cuttings. Photo credit: Thomas A. Dudek, MSUE

Fungus gnat adult vs. Shore fly.
Photo 2. Fungus gnat adult vs. Shore fly. Photo credit: Tina Smith, U Mass Extension

Fungus gnat larvae.
Photo 3. Fungus gnat larvae.
Photo credit: Leanne Pundt, U Conn Extension

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