revised January 2006

Fungus Gnats are Serious Pests

fig. 1 - adult fungus gnatFungus gnats were once thought to be a nuisance pest only in the greenhouse. However, as fungus gnats were shown to cause direct feeding damage to crops, they are now considered a serious pest.

IDENTIFICATION
Fungus gnats and shoreflies (figures 1 & 2) both thrive in the damp, moist greenhouse environment. Correct identification is needed because management tactics vary according to the insect present .

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 an Y-shaped vein in the wing pattern. (figure 3) Adults tend to fly in a zig-zag pattern and are attracted to fungi so might be fig.2  shorefly adultobserved near plants with Botrytis sporulation. Females lay their eggs nearby so the larvae have access to a fungal food source. Fungus gnat larvae are small, translucent to white in color with a distinctive black head capsule (Figure 4).

Shore fly adults resemble a small housefly with stockier bodies, plus shorter legs and antennae than fungus gnats. Shore fly larvae are white, wedge-shaped and do not have a distinctive head capsule. Larvae may be found near algae, a primary food source. They do not feed on plants.

fig 3. y-shaped wing veinFUNGUS GNAT DAMAGE
Fungus gnats are most damaging to seedlings, cuttings and young plants. As fungus gnat larvae feeding on tender young roots, this feeding injury  provides an entryway for pathogens. Larvae also feed on the developing callus of direct stuck cuttings, delaying rooting. Fungus gnats are general feeders. Plants with succulent stems, such as geraniums, sedum, coleus and poinsettias, are especially prone to injury and suffer serious losses. As the young feeder roots and stems are damaged, affected plants wilt. Leaves may turn yellow and drop. In laboratory studies, adult fungus gnats carried spores of Pythium, Botrytis, Verticillium, Fusarium and Thielaviopsis as they moved from plant to plant. Spores have also been found in their droppings. It is unclear how important this disease transmission is in commercial greenhouses.

fig. 4 fungus gnat larvaeLIFE CYCLE
The fungus gnat's life cycle from egg to adult may be completed in as little as three to four weeks depending on temperature. Eggs are laid in cracks and crevices in the media surface and mature in four to six days. Fungus gnat larvae feed and develop for about two weeks at 72oF. Pupation occurs in the soil. After four to five days, adults emerge. Overlapping and continuous generations make control difficult.

CULTURAL CONTROLS
Adults are attracted to newly planted crops, making it important to thoroughly clean the greenhouse before introducing new crops. Dry, level, weed-free, well-drained floors help eliminate breeding areas. Keeping compost piles away from the greenhouse and cleaning up any spilled media on the floor also helps eliminate breeding areas.

Inspect incoming plugs for fungus gnat larvae or their feeding damage.  Recent studies have shown that fungus gnats may be introduced into a greenhouse from  soilless media or rooted plant plugs. 

 

Adults are attracted to mixes with high microbial activity, or with high amounts of peat moss or hardwood bark. Avoid using mixes with immature composts less than one year old. However, no potting mix is completely immune to fungus gnat infestations.  Adult females prefer to lay their eggs in protected, humid crevices in the media. How the media is handled and stored may be more important than the type of mix used. If the mix is stored outside and stays moist, it may support more fungus gnat activity. Tears or openings in the bags enable resident fungus gnat populations to gain entry into the media bags. Store the media so that it stays dry.

MONITORING
Monitoring is especially crucial if you are planning on targeting biological controls or insect growth regulators against the fungus gnat larvae. Inspect incoming plugs for fungus gnat larvae or their damage.  Place yellow sticky cards in samples of growing media to monitor for any emerged adults.  fig. 5 potato plug

Yellow sticky cards, placed horizontally at the soil surface, can be used to detect adults. Check and change the cards weekly to determine population trends. Use potato plugs (at least one inch in diameter) placed on the soil surface to monitor for larvae (Figures 5 and 6). When using potato plugs, place the plug so there is contact with the media to ensure that the potato plug does not dry out. To look for larvae, first check the growing media under the plug and then the surface of the potato itself.  Check the potato plugs after 48 hours.  Be sure to mark the pots where you placed the potato plugs, so you can easily find them! If not removed, potato chunks can "melt out," sprout or be fed upon by mice.  For smaller cuttings or plugs, potato slices, resembling a “French fry” can be placed in the media.

BIOLOGICAL CONTROLS
Bacillus thuringiensis var. israelensis, sold under the trade name of Gnatrol, is most effective against the young first instar larvae. The bacteria must be ingested by the larva, after which a toxic protein crystal is released into the insect's gut. Larvae stop feeding and die. Gnatrol is only toxic to larvae for two days. Repeat applications, i.e. two or three applications at high rates, may be needed to provide effective control.

Steinernema feltiae  (ScanMask, NemaShield, Nemasys or Entonem) are beneficial, insect killing nematodes that are also applied as a drench treatment against fungus gnat larvae.  After entering the target insect through various openings, the nematodes multiply within the host and release a bacterium whose toxin kills the larvae.  These beneficial nematodes reproduce within the fungus gnat larvae; exit the dead body and search for new hosts to infect. Fungus gnat larvae are killed in one to two days.  Early treatments of rooted plant plugs and cuttings with beneficial nematodes provide better control than later treatments. 

A small, soil-dwelling predatory mite, Hypoaspis miles, feeds on fungus gnat larvae. It is shipped in a vermiculite/peat carrier with all stages of the predatory mites. The vermiculite/peat carrier can be distributed over the media surface, especially when pots are placed close together.  These predatory mites are best used when fungus gnat populations are low and are compatible with Gnatrol and S. feltiae.

INSECT GROWTH REGULATORSfig. 6 close up larvae on potato plug
Insect growth regulators (IGR’s) are most effective against the young developing larvae and will have no direct activity against adults. Commercially available IGR's include Adept, Azatin XL, Aza-Direct, Ornazin, Citation, Distance and Enstar II. Adept (difluorobenzamide) is an insect growth regulator that is labeled for fungus gnat and shore fly larvae. In 1998, New England growers reported marginal leaf burn on poinsettia crops. The injury seemed to be rate related, especially if the volume of drench applied exceeded the recommended labeled rates. Injury symptoms did not occur until four to six weeks after the application. Please note that the Adept label has been changed. The label now reads:" Warning: Exceeding label rates, volumes or number of applications of Adept can cause serious foliar injury to crops, particularly to poinsettias, hibiscus and Reiger Begonias. Do not apply Adept to poinsettias, hibiscus and Reiger Begonia. No sensitivity has been observed with other bedding or pot crops including plugs and liners, however, tests should be conducted to insure safety to any crop before extensive use."

Other Treatment Options

DuraGuard ME is a microencapsulated product that may be used for both larvae and adults.  The neonictotindoids Safari (dinotefurnon), Flagship (thiamethoxam), Marathon (imidacloprid) and Tristar (acetamiprid) are labeled for fungus gnat larvae.  Pylon has contact and stomach activity against fungus gnat larvae.

 

Treatments Against Adults

For well-established populations, applications of an adulticide may be of benefit. Some insecticides for use against fungus gnat adults include Talstar Flowable, Attain TR, Decathlon 2O WP, Astro, 1300 Orthene TR, 1100 Pyrethrum TR, Duraplex TR, and Tame/Orthene TR. Contact materials such as insecticidal soap, ultra-fine oil, and synergy super fine spray oil emulsion are also labeled for fungus gnat adults.

 

Mention of particular materials is for educational purposes only and is not to be interpreted as an endorsement, nor is criticism implied of any materials not mentioned.

Consult and follow labels for registered uses.  To avoid any potential phytotoxicity problems, spot test first before widespread use.

REFERENCES:

 

Leanne Pundt, Extension Educator and Greenhouse IPM Coordinator, Commercial Horticulture, University of Connecticut
Published in Yankee Grower. 1999. September/October. p. 9-10.

Revised January 2006.

 

More on Fungus Gnats & Shore flies
More Fungus Gnats & Shore flies Images

Information on our site was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.

 

Information on our site was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.

The information in this material is for educational purposes. The recommendations contained are based on the best available knowledge at the time of printing. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension system does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available.All agrochemicals/pesticides listed are registered for suggested uses in accordance with federal and Connecticut state laws and regulations as of the date of printing. If the information does not agree with current labeling, follow the label instructions. The label is the law.Warning! Agrochemicals/pesticides are dangerous. Read and follow all instructions and safety precautions on labels. Carefully handle and store agrochemicals/pesticides in originally labeled containers immediately in a safe manner and place. Contact the Connecticut Department of Environmental Protection for current regulations.The user of this information assumes all risks for personal injury or property damage.Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Kirklyn M. Kerr, Director, Cooperative Extension System, The University of Connecticut, Storrs. The Connecticut Cooperative Extension System offers its programs to persons regardless of race, color, national origin, sex, age or disability and is an equal opportunity employer.

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