Familiarize yourself with fungus gnats

Focus on Pest Control - Fungus gnats

From tunneling larvae to flying adults, here’s what growers need to know about this greenhouse pest.

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December 30, 2019

Signs of fungus gnat larvae chewing on leaves. Look under the leaves for the larvae.
Photos courtesy of Leanne Pundt

Fungus gnat adults (Bradysia spp.) are dark-bodied, slender, long-legged insects in the order Diptera. They are a common pest, especially in the moist growing environment in propagation greenhouses. Larvae feed upon the developing roots of cuttings and young plants.

Fungus gnat larvae feed upon young cuttings and plugs, causing root injury and plant wilting. Larvae can tunnel into plant crowns causing plant death. Larvae also create wounds that can allow the entry of soilborne pathogens. Fungus gnat adults and larvae may also spread soilborne plant pathogens such as Thielaviopsis, Phytophthora, Pythium and Fusarium.

Fungus gnat larvae are general plant feeders. Ornamental plants with succulent stems, such as begonias, geraniums, sedum, coleus and poinsettias, are especially prone to injury, especially if the growing media contains a high percentage of compost, composted bark or peat moss.

Hunter fly adult
Fungus gnat larvae and damage

Scouting for fungus gnats

Use yellow sticky cards placed horizontally at the media surface to attract fungus gnat adults. Adult fungus gnats are small (1/8-inch-long), mosquito-like insects, with long legs and antennae. Their two wings are delicate and clear with a Y-shaped vein in the wing pattern. Check yellow sticky cards weekly. For more, visit here.

Place potato chunks or plugs on the media surface to attract fungus gnat larvae. Check potato slices after two days for the larvae. Fungus gnat larvae are small, (approximately ¼ of an inch long when mature), translucent to white in color with a distinctive black head capsule. Inspect root systems for overall health and for signs of damage from fungus gnat feeding (i.e. blunt root tips).

Fungus gnat adult on sticky card
Fungus gnat larvae

Biological control options for fungus gnats

Fungus gnat larvae can be effectively managed using entomopathogenic or insect-killing nematodes, Steinernema feltiae (NemaShield, Nemasys, Scanmask, Entonem) that are applied as a preventive soil drench or sprench. They are best applied to moist soils, in the early morning or evening to avoid high temperatures and ultraviolet levels. Repeat applications every two weeks against fungus gnat larvae. Beneficial nematodes can be used alone, or in combination with other biological control agents. Growers just beginning to use biological control agents, often start with S. feltiae. This is because they work so well and can also be used in rotation with insecticides. Be sure to check with your biological control supplier for information on compatibility with insecticides and specific guidelines for application. For example, visit here.

Soil-dwelling predatory mites (Stratiolaelaps scimitus, formerly known as Hypoaspis miles), predatory rove beetles (Dalotia coriaria) and entomopathogenic nematodes (Steinernema feltiae) can all be used in a preventive biological control program against fungus gnat larvae. The growing medium should be moist before applying these natural enemies. Although not commercially available, predatory hunter flies (Coenosia attenuate) may be introduced on incoming plant material. Biological controls are best used preventively combined with good cultural controls: avoiding overwatering, removing low-growing weeds and having a clean, dry greenhouse.

Insecticides that control fungus gnats

There are a number of insecticides available for use against fungus gnat larvae, which is the life stage to target. Repeated applications are often needed. Some materials include insect growth regulators: cyromazine (Citation) (MOA 17), azadirachtin (Azatin, Molt-X), pyriproxyfen (Distance) (MOA 7c) and diflubenzuron (Adept) (MOA 15). The microbial insecticide, Bacillus thuringiensis subsp. israelensis (Gnatrol WDG) (MOA 11A) works best against the young, first instar larvae. Chlorfenapyr (Pylon) (MOA13), dinotefuran (Safari) (MOA4A) or thiamethoxam (Flagship)(MOA4A) may also be options. Read labels carefully for plant safety precautions and follow resistance management guidelines.

Consult and follow pesticide labels for registered uses: local restrictions may apply. No discrimination is intended for any products not listed. If any information is inconsistent with the label, then follow the label.

The author is an extension educator at UConn Extension specializing in Greenhouse IPM. She can be reached at leanne.pundt@uconn.edu