Treatments for fungus gnats 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 dissolves their internal contents.
The predatory beetle Dalotia (=Atheta) (rove beetle) may be applied during seeding or sticking cuttings. Dalotia 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. Growers can create their own distribution boxes to rear the rove beetles in the greenhouse.
Bacillus thuringiensis subsp israelensis (Gnatrol) is a stomach poison that is applied as a drench. It must be ingested by the fungus gnats and is most effective against the young, first instar larvae, so repeated applications are 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, Molt-X, Ornazin, Azera, AzaSol), cyromazine (Citation) and pyriproxyfen (Distance) against the fungus gnat larvae. Chlorfenapyr (Pylon) has contact and stomach activity against fungus gnat larvae.
Treatments should also be applied under greenhouse benches where fungus gnats often thrive.
Not all the products that are effective against fungus gnats will work against shore flies. The beneficial nematode Steinernema carpocapsae (Millenium) is used to manage shore flies. IGRs such as azadirachtin, diflubenzuron, cyromazine and pyriproxyfen (Read and follow plant safety precautions on the label) can also be used for shore fly larvae. 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.
Tina Smith, UMass Extension and Leanne Pundt, UConn Extension
On Farm Rearing of the Soil Predator Dalotia coriaria MSU Organic Pest Management Laboratory
Fungus Gnats, UConn Extension
Biological Control of Fungus Gnats, UConn Extension
Shore Flies in the Greenhouse, UConn Extension
Fungus Gnats and Shore Flies, UMass Extension