Taking a more holistic approach to fungus gnat management

Cloudy, gray, damp weather coupled with overly damp soilless media create the perfect environment for fungus gnat outbreaks in greenhouse operations. Insecticides that target the adults can be deployed for a quick knockdown, but a more holistic program utilizing biocontrol agents may work better for growers in the long haul.

There are three major biocontrol agents (BCA) that can be utilized by greenhouse growers to control fungus gnat larvae. These BCAs include a predaceous mite, Strateolaelaps scimitus, the rove beetle (Atheta coriaria) and a nematode, Steinernema feltiae. All three of these bicontrol agents can be quite effective if deployed early in the cropping cycle.

feeds on first instar fungus gnat larvae, overwintering two-spotted spider mites, and thrips pupae. Strateolaelaps should be introduced early in the cropping cycle before the fungus gnat populations become too abundant. As a rule, best results are obtained when trap counts on yellow sticky cards are less than 10 per week. Two applications of Strateolaelaps scimitus 2-3 weeks apart usually provides excellent control.

Atheta coriaria, the rove beetle, is a soil-dwelling insect that feeds on a wide array of small insects and mites. Atheta is relatively slow to establish and should be introduced into the cropping cycle early when less than 10 fungus gnat adults are being caught per week on yellow sticky cards. Atheta coriaria is not considered a strong biocontrol agent on its own against fungus gnats and should be used in concert with Strateolaelaps scimitus releases.

Steinernema feltiae is an entomopathogenic nematode that, when released into the media, will seek out and enter the insect though a natural opening in the insect (mouth, anus or spiracle).

Once inside the insect, the nematode releases a symbiotic bacteria into the insect’s system, resulting in its death in 24-48 hours. Beneficial nematodes are typically applied at a rate of 50 ml per 1,100 square feet to pre-moistened media. Growers will enjoy greater success with Steinernema feltiae and other nematodes if they shut off fans after application (to lessen desiccation). Growers should also refrain from applying beneficial nematodes under high light conditions.

NoFly WP (Isaria fumosoroseus Strain FE 9901), a mycoinsecticide, is labeled for controlling both adult and larval fungus gnats. NoFly works best when the relative humidity can be maintained at 80% for 8-10 hours in the greenhouse after application. Mycoinsecticides can be an effective tool in the greenhouse, but they sometimes can kill other BCAs that have been released by the grower. Before utilizing a broad spectrum mycoinsecticide in the growing environment, please consult with your BCA supplier to see if they will impact your biocontrol program.

Another bio-based product that can be used to control fungus gnats is Gnatrol WDG (Bacillus thuringiensis subsp. israelensis). It can be used as a drench to control fungus gnat larvae in media and works well if it is not combined with fertilizers or fungicides containing copper or chlorine. Fungus gnat larvae must ingest the material to be killed.

Additional larvicides for managing fungus gnat larvae include azadirachtin (Azatin XL, Molt-X), cyromazine (Citation), s- kinoprene (Enstar), diflubenzuron (growers should not use Adept on poinsettias, hibiscus or Rieger begonias), dinotefuran (Safari 20 SG), imidacloprid (Benefit 60 WP, Marathon 60 WP, Mallet 2F T & O, Marathon II, Marathon 1% G), nematodes: Steinernema carpocapsae (Millenium), Steinernema feltiae (Entonem, Nemasys, Nemashield, Scanmask), pyriproxyfen (Distance or Fulcrum) (Do not use on salvia, coral bells, or on some foliage plants), and thiamethoxam (Flagship 25 WG).

Insecticides for controlling adult fungus gnats include acephate (1300 Orthene TR), acetamiprid (Quasar 8.5 SL or TriStar 8.5 SL), bifenthrin (Ascertain TR, Attain TR, Talstar P, Talstar Select), canola oil and pyrethrins (Pycana), chlorfenapyr (Piston or Pylon), Cyfluthrin (Decathlon 20 WP), fenpropathin + acephate (Tame/Orthene TR), horticultural oil (SuffOil –X, Ultra-Pure Oil, etc.), imidacloprid + cyfluthrin (Discus L), permethrin (Astro), pyrethrin (Pyganic EC 5.0 II), and pyrethrins + Beauveria bassiana Strain GHA (BotaniGard Maxx).

Ford currently serves as a Commercial Horticulture Educator with Penn State Extension and is housed in the Cambria County Extension Office in Ebensburg, Pennsylvania. He has a B.S. degree in Ornamental Horticulture from the University of Maryland and a MBA from Frostburg State University in Frostburg, Maryland.