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Greenhouse Updates: Feb 13, 2015

Fungus Gnats and Shore Flies
February 13, 2015

Fungus gnats and shore flies are a common problem in propagation houses, especially on slow growing seedlings and plants. Moist growing media encourages the growth of algae on the media surface providing an ideal environment for their development. Adult fungus gnats are also attracted to newly planted crops, where the females lay their eggs in the crevices of the moist growing media. Eggs hatch into larvae that feed upon tender, young roots; the developing callus of cuttings, and even bore into the stems of succulent plants such as sedum, coleus and geraniums.

Dry, level, weed-free, well-drained floors help eliminate breeding areas under the greenhouse benches.

To monitor fungus gnat and shore fly adults, use yellow sticky cards, placed horizontally at the soil surface. Check and change the cards weekly to see if numbers are increasing or decreasing. Use chunks of peeled potatoes to monitor for fungus gnat larvae. Mark the containers where you have placed the potatoes so you can easily find them! Check the potato after 48 hours for the presence or absence of larvae.

Fungus gnat management: Treatments are best directed against the larval stage. Many growers use beneficial nematodes Steinernema feltiae sold under the trade names of Nemashield, Nemasys, Entonem or Scanmask applied as a drench. Fungus gnat larvae are killed in one or two days by the symbiotic  bacterium that kills fungus gnat larvae by dissolving their internal contents.  The predatory beetle  Atheta = Dalotia  (rove beetles) are  also used, applied during seeding or sticking cuttings. Atheta are nocturnal and are easily established in greenhouses. Open the container where you will release them and sprinkle the beetles evenly across the area to be treated. Reapply when transplanting or repotting. Another product for fungus gnat larvae is Bacillus thuringiensis subsp israelensis (Gnatrol) which is a stomach poison that is applied as a drench. It must be ingested and is most effective against the young, first instar larvae, so repeated applications may be needed to provide effective control.

Many growers also report good results when using insect growth regulators (IGRs) such as diflubenzuron (Adept), azadirachtin (Azatin XL, Azatin O, Aza-Direct, AzaGuard, Azahar, Molt-X, Ornazin), cyromazine (Citation), pyriproxyfen (Distance) and s-kinoprene (Enstar AQ) against the fungus gnat larvae.

Chlorfenapyr (Pylon) has contact and stomach activity against fungus gnat larvae.

If fungus gnats are breeding in the areas under the greenhouse benches, treatments should be applied to those areas, too.

Shore fly management: Not all products that are effective against fungus gnats will work against shore flies.  The beneficial nematode Steinernema carpocapsae (Millenium) is sold to manage shore flies.  IGRs such as azadirachtin, diflubenzuron (Adept), cyromazine (Citation) and pyriproxyfen (Distance) (Read and follow plant safety precautions on the label) can be used for shore fly larvae. However Bacillus thuringiensis (Gnatrol) is ineffective. Pyrethroid-based insecticides and spinosad (Conserve) are labeled for adult shore flies. Growers may eventually need both an adulticide and a larvicide for established populations.

Fact Sheets:
Fungus Gnats and Shore Flies, UMass Extension
Fungus Gnats, UConn Extension
Biological Control of Fungus Gnats, UConn Extension
Shore Flies in the Greenhouse, UConn Extension

Leanne Pundt, UConn Extension and Tina Smith, UMass Extension