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.
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.
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.
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.
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.
|Manufacturer||REI1||Mode of action2||Comments|
|biological||A.||Bacillus thuringiensis ssp. israelensis#
|Valent||4||11||Do not apply with fertilizers or fungicides containing copper or chlorine. Not effective on shore flies.|
(PT Pyrethrum TR)
|12||3/—||An aerosol. Also effective against adults.
|insect growth regulator||A.||azadirachtin
||Must contact insect. Repeat applications as necessary. Only effective on larvae. Label permits low-volume application.|
|SePRO||12||un||Do not exceed 22.5 oz/acre/application|
(Citation 75 WP)
|Syngenta||12||17||Certification training required to use this product. Also effective against shorefly larvae.|
|Chemtura||12||15||Apply as spray or drench to top 2 inches of soil.|
|Valent||12||7C||Do not apply more than 2 times per cropping cycle or per 6 months.|
|Wellmark||4||7A||Apply prebloom. Also labeled for low volume use.|
|Cleary||12||4A||Apply as a foliar spray.
|Valent||12||4A||Can be applied as a drench or foliar spray.|
|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.|
|Syngenta||12||4A||Can be applied as a drench or foliar 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.|
(PT 1300 Orthene TR)
|Whitmire MicroGen||24||1B||An aerosol for greenhouse use only.|
|Whitmire MicroGen||12||3||Check label. A fogger for greenhouse use only.|
|FMC||12||3||Also effective against adults. Label permits low-volume application.|
(Decathlon 20 WP)
|OHP||12||3||Also effective against adults. Label permits low-volume application.|
(Tame 2.4 EC Spray)
|Valent||24||3||Also effective against adults. Label permits low-volume application.|
|Wellmark||12||3||Also effective against adults. Label permits low-volume application. Also labeled as a cutting dip at 5 fl oz/100 gal.|
|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.|