Agriculture: Floriculture and Ornamental Nurseries Pest Management Guidelines

Fungus Gnats

  • Fungus Gnats: Bradysia coprophila, Bradysia impatien
  • Description of the Pest

    Fungus gnats are small (2–5 mm long) mosquitolike flies with dark wings, delicate legs, and long antennae. They lay their eggs in soil, and the eggs hatch about 4 days later. There are four larval instars that increase in size up to about 0.33 inch (8 mm). Larvae are clear, with visible internal organs, and have shiny black head capsules. Initially larvae feed on root hairs and algae; later, larvae may feed on the insides of roots. When populations are high, larvae may bore into larger roots or stems that are in the soil. Larvae will also feed on leaves touching the soil. One generation may complete development in 21 (72°F) to 40 (61°F) days.

    Damage

    Larvae usually feed on roots and algae within 1 inch of the soil surface. Root feeding by larvae can allow entry of plant pathogens. Direct damage through root feeding can cause wilting even though the plants are being sufficiently watered. Damage is particularly severe in propagation areas, in seedling flats, and with especially sensitive crops. Adult fungus gnats also disseminate soil-inhabiting pathogens on their bodies and in their feces. Fungus gnat adults can be a nuisance when present in large numbers.

    Management

    Biological Control

    Biological control agents include nematodes (Steinernema feltiae), soil-inhabiting predaceous mites (Hypoaspis miles), and the bacteria Bacillus thuringiensis (Gnatrol). For more information, see BIOLOGICAL CONTROL.

    Cultural Control

    Keep production areas free of weeds and algal scum, which can serve as breeding sites for fungus gnat populations. Maintaining overwatered conditions and using either incompletely composted organic matter or manure in potting media provides ideal conditions for fungus gnats. Commercial sources of peat may be infested with fungus gnats and should be steamed before use when growing crops sensitive to fungus gnats.

    Monitoring and Treatment Decisions

    Yellow sticky cards placed in greenhouses will capture adult fungus gnats. For more information, see MONITORING WITH STICKY TRAPS. Small emergence traps can also be used to determine precisely where adults are emerging. Larval populations can be monitored with cubes or slices of potatoes pressed just into the soil. Fungus gnat larvae can be readily seen feeding on the potato pieces. For more information on treatment decisions, see ESTABLISHING ACTION THRESHOLDS.

    Apply insecticide drenches to the top 1 inch of soil to kill larvae; avoid applying excessive spray volume that may leach or move insecticide too deeply into growing media. Pyrethrins and other adulticides such as aerosols, foggers, or sprays can quickly, but temporarily, reduce adult fungus gnat numbers.

    Treatment

    Selected Materials Registered for Use on Greenhouse or Nursery Ornamentals

    Read and follow the instructions on the label before using any pesticide. Before using a pesticide for the first time or on a new crop or cultivar, treat a few plants and check for phytotoxicity. Also consider pesticide resistance management and environmental impact.

    Class   Pesticide
    (commercial name)
    Manufacturer REI1 Mode of action2 Comments
    biological A. Bacillus thuringiensis ssp. israelensis#
    (Gnatrol)
    Valent 4 11 Do not apply with fertilizers or fungicides containing copper or chlorine. Not effective on shore flies.
    botanical A. pyrethrin/PBO3
    (PT Pyrethrum TR)
    Whitmire
    MicroGen
    12 3/— An aerosol. Also effective against adults.

    insect growth regulator A. azadirachtin
    (Azatin XL)

    OHP

    4

    un

    Must contact insect. Repeat applications as necessary. Only effective on larvae. Label permits low-volume application.
    B. azadirachtin
    (Ornazin 3%EC)
    SePRO 12 un Do not exceed 22.5 oz/acre/application
    C. cyromazine
    (Citation 75 WP)
    Syngenta 12 17 Certification training required to use this product. Also effective against shorefly larvae.
    D. diflubenzuron
    (Adept 25WP)
    Chemtura 12 15 Apply as spray or drench to top 2 inches of soil.
    E. pyriproxyfen
    (Distance)
    Valent 12 7C Do not apply more than 2 times per cropping cycle or per 6 months.
    F. s-kinoprene
    (Enstar II)
    Wellmark 4 7A Apply prebloom. Also labeled for low volume use.
    neonicotinoid A. acetamiprid
    (TriStar) 70WSP
    Cleary 12 4A Apply as a foliar spray.
    B. dinotefuran
    (Safari) 20G
    Valent 12 4A Can be applied as a drench or foliar spray.
    C. imidacloprid
    (Marathon 1G)
    (Marathon II)
    OHP 12 4A Not to be used more than once every 16 weeks. Do not apply to soils that are water logged or saturated. Do not apply to bedding plants intended to be used as food crops.
      (Marathon 60 WP)       As above. Apply only as a drench.
    D. thiamethoxam
    (Flagship) 25WG
    Syngenta 12 4A Can be applied as a drench or foliar spray.
    organophosphate A. acephate
    (Acephate 97UP)
    United Phosphorus 24 1B  
    B. acephate
    (Orthene T,
    T&O Spray)
    Valent 24 1B A number of chrysanthemum varieties have exhibited phytotoxic reactions. In greenhouses only labeled for use on anthurium, cacti, carnation, rose, orchids, some foliage plants, young poinsettia, and some varieties of chrysanthemum. Can stunt new growth in roses.
    C. acephate
    (PT 1300 Orthene TR)
    Whitmire MicroGen 24 1B An aerosol for greenhouse use only.
    pyrethroid A. bifenthrin
    (Attain TR)
    Whitmire MicroGen 12 3 Check label. A fogger for greenhouse use only.
    B. bifenthrin*
    (Talstar Professional)
    FMC 12 3 Also effective against adults. Label permits low-volume application.
    C. cyfluthrin
    (Decathlon 20 WP)
    OHP 12 3 Also effective against adults. Label permits low-volume application.
    D.
    deltamethrin*
    (DeltaGard)
    Bayer 12 3  
    E. fenpropathrin*
    (Tame 2.4 EC Spray)
    Valent 24 3 Also effective against adults. Label permits low-volume application.
    F. fluvalinate
    (Mavrik Aquaflow)
    Wellmark 12 3 Also effective against adults. Label permits low-volume application. Also labeled as a cutting dip at 5 fl oz/100 gal.
    G. permethrin
    (Astro)
    FMC 12 3 Direct application to blooms may cause browning of petals. Marginal leaf burn may occur on salvia, diffenbachia, and pteris fern. Label permits low-volume application. Do not apply more than 2 lb a.i./acre/year.
    1 Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing.
    2 Rotate chemicals with a different mode-of-action Group number, and do not use products with the same mode-of-action Group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a Group number of 1B; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode of action Group numbers are assigned by IRAC (Insecticide Resistance Action Committee).
    3 PBO = piperonyl butoxide
    * Restricted use pesticide. Permit required for purchase or use.
    # Acceptable for use on organically grown ornamentals.
    Text Updated: 03/09
    Treatment Table Updated: 03/09