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Managing Fungus Gnats and Shore Flies

February 15, 2011

Growers are all too familiar with the presence of the numerous small black flies that frequently occur during the production of greenhouse and nursery crops. These flies are most commonly fungus gnat (Bradysia coprophillia and Bradysia impatiens) or shore fly (Scatella stagnalis) adults.

Although fungus gnats and shore flies are mostly considered a nuisance, they can cause injury to crops either directly or indirectly. Through their feeding activity, fungus gnat larva frequently causes injury to root systems; this is particularly problematic during propagation. Otherwise, all life stages of both of these pests indirectly cause injury to plants by their transmission of diseases (Botrytis, Cylindrocladium, Fusarium, Pythium, Phytopthora, Thielaviopsis, and Verticillium) either on their bodies or fecal deposits.

Description and Life Cycle

Adult fungus gnats are small 1/8 in. long, slender, dark brown to black flies with long and dangling legs, resembling those of a mosquito and have a distinct ‘Y’ shaped vein on each wing. They are weak fliers and are usually observed flying at or near the surface of the growing medium. The larvae measure 1/4 to 1/2 in. long, have translucent bodies with a visible digestive tract, and a black shiny head capsule. From egg to adult, it typically takes 3 to 4 weeks to complete the entire life cycle.

Adult shore flies have black bodies that resemble miniature houseflies measuring 1/8 in. long. They have stout bodies with short legs, short antennae, reddish eyes, and dark wings with five pale spots. Compared to fungus gnats, adult shore flies are strong flyers. Shore fly larvae resemble small, opaque yellowish brown maggots measuring 3/8 in. long and do not have a head capsule. The life cycle from egg to adult is 14 to 18 days.

Fungus gnats thrive in moist conditions, especially during propagation or before plants establish well developed root systems. Shore flies also tend to be attracted to moist areas, particularly when algae growth is present. At any given time, there is usually multiple life stages present with both of these pests.

Monitoring and Control Methods

Adult fungus gnats and shore flies can be detected by placing yellow sticky cards 1 to 2 inches above the crop canopy and the growing medium. It is recommended to place 1 sticky card per 1000 to 3000 square feet of production space. Inspect the sticky cards weekly to observe the number of adults present. A trained eye can quickly distinguish between fungus gnat and shore fly adults. These counts can be used to determine if the number of adults is increasing or decreasing. Monitoring the adult population will provide an indication of whether an infestation is about to occur or if the current levels are non-threatening to the crop.

Reducing free-standing water and controlling algae is the first step in fungus gnat and shore fly management. Controlling the amount of moisture in the production site greatly reduces the presence of algae, which provides both a food source and a breeding ground for these insect pests. Also avoid applying excessive amounts of water to the crops, as it is important to allow the top layers of growing medium to become dry between irrigations. It is beneficial to control algae growth in the production area, such as on bench surfaces, walls, and pathways using disinfectants containing hydrogen dioxide or quaternary ammonium compounds. It is also recommended to discard any dead or diseased plant materials, debris, and weeds which are located in the production site.

The best strategy for controlling fungus gnats and shore flies involves controlling them while they are in the larval stages. There are a number of commercially available insecticides to control fungus gnat and/or shore fly larva including azadirachtin, chlorfenapyr, chlorpyrifos, cyromazine, diflubenzuron, and pyriproxyfen. Most of these chemicals are insect growth regulators, which often mimic the juvenile growth hormone, causing them to molt prematurely or preventing them from entering their next stage of development. These products provide little to no control for the adult population.

There are a number of effective insecticides which provide ‘knock down’ control of the adult populations including bifenthrin, cyfluthrin, lambda-cyhalothrin, pyrethrins, and permethrin. These products do not control the eggs or pupal life stages, and may require multiple applications to provide sufficient levels of control.

Paul Pilon
Perennial Solutions Consulting
paul@perennialsolutions.com

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