Biological Control of Fungus Gnats  

Fungus gnats are a common greenhouse pest, especially in the moist environment common in propagation houses. Fungus gnat larvae feed upon young cuttings and plugs, causing root injury and death. Fungus gnats help spread pathogens including Pythium, Botrytis, Fusarium and Thielaviopsis.   

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 on roots or stems at the base of the plant and develop for about two weeks at 72^oF. Pupation occurs in the soil. After four to five days, adults emerge. Overlapping and continuous generations make control difficult. Control may be especially challenging with favored plant species, such as poinsettias, or if the growing media contains bark or peanut hulls, where they like to hide.   

Biological controls are best used preventively, when populations are low. A regular monitoring program is needed for early detection of this pest and to insure the success of a biological control program. Yellow sticky cards can either be placed horizontally at the media surface or laid flat on the rims of pots to capture resting adults. Potato chunks or plugs (peeled potatoes cut in quarters or as a French fry for plugs) can be placed on the media surface to attract larvae. Check yellow sticky cards weekly and inspect the potatoes after 2 days. Regular inspection of developing root systems for signs of fungus gnat feeding (blunt root tips) is also helpful. Cultural controls (avoiding overwatering, avoiding puddling on the floors, rigorous weed controls, and controlling algae) are critical before starting a biological control program for fungus gnats.  

There are a number of biological control agents that may be incorporated into your pest management program for fungus gnats. These include entomopathogenic nematodes, soil dwelling predatory mites, and rove beetles. In addition, a specific strain of Bacillus thuringiensis, subsp. israelensis (sold under the trade name, Gnatrol ™) is commercially available for use against fungus gnat larvae.  Although not yet commercially available, predatory hunter flies, have been found in greenhouses from Canada to Texas and if present, may help reduce fungus gnats. 

Entomopathogenic Nematodes  

Nematodes are small, colorless, unsegmented, cylindrical round worms that occur naturally in soils throughout the world. The term "entomopathogenic" means to cause diseases ("pathogenic") to insects ("entomon" from the Greek word for insect).  Insect parasitic nematodes are primarily found in the families Steinernema and Heterorhabditis. Different species work best against different pests. S. feltiae is primarily used against fungus gnat larvae, and thrips pupae in the soil.  Fungus gnat larvae may be parasitized in any larval stage. Nematodes do not swim, so are not effective against aquatic shore flies. Heterorhabditis is primarily used against black vine weevil larvae.  

The nematode's life cycle consists of eggs, four larval stages and the adults.   The third instar juvenile stage is the infective stage. This stage searches out susceptible host insects  by detecting excretory products, carbon dioxide and temperature changes. The nematodes enter the insect host through body openings. They multiply within the host and release a symbiotic bacterium (Xenorhabdus) whose toxin kills the fungus gnat larvae.  The larvae are killed in one to two days by septomecia (blood poisoning).  More than one generation of nematodes may develop in dead host insect. The infective juveniles then exit the dead body and search for new hosts to infect. 

The nematode S. feltiae is sold under the trade names of  NemaShield, Nemasys or Entonem which are applied as a soil drench treatment against fungus gnat larvae. Preventative applications to moist soils work best.  Apply nematodes with a sprayer (remove screens and filters), injector, hose end sprayer or even a watering can. If using an injector, set the dilution to 1:100. Remove all filters or screens on the intake tube. Remove pump filters. The formulation of S. feltiae sold as ScanMask is applied mulch or topdressing to the growing media.  

Unlike many traditional pesticides there is no REI (an added bonus in propagation houses), nor possibility that the fungus gnat larvae will develop resistance. No adverse effects have been shown against non-target organisms in many different field studies. But, beneficial nematodes are living organisms, so there are a number of precautions you need to follow for their successful use. Like any biological control agent, they are best used preventively, before an outbreak occurs.  

Tips for Use            

• Treat as soon as possible (2 to 3 days) after sticking cuttings, planting plugs or starting seeds. 
• Check viability before application

To do this, place a small amount of the product in a small container or petri dish. Add 1 or 2 drops of room temperature water; wait a few minutes and look for actively moving or swimming nematodes. 

Use a dark black background and a hand lens or field microscope to see the small (0.6 mm or 0.02 inches in length) nematodes.

• Apply as a media drench to target the fungus gnat larvae.
• Media temperatures should be above 50° F but avoid applying when soil temperatures are above 80°F. Optimum media temperatures are between 60-70°F. (Use a soil thermometer to monitor temperatures).
• Water the growing media before and after application. (Nematodes need moisture for movement). But, avoid over watering, so they aren’t washed out of the container.
• Apply in the evening or at dusk or on a cloudy, overcast day. (Nematodes are very sensitive to UV light and desiccation).
• Nematodes are compatible with a number of different pesticides.

However, they are generally not compatible with organophosphates, carbamates, and nematicides. 

• For more detailed information on pesticide compatibility: consult with your supplier or with the following resources on the Internet:

- Pesticide Side Effects Database – www.koppert.com (accessed 9/07)

- Pesticide Side Effects Database - www.biobest.be/ (accessed 9/07)

• Apply immediately after receiving them, if possible.  If you must store the nematodes, store in a refrigerator (38-42°F). Avoid placing in a small refrigerator where they may freeze.
• Check expiration date on the package for how long you can store them.
• Repeated applications are often needed. Reapply in 2 to 4 weeks under moderate to heavy infestations. For longer term crops, apply at the beginning and at mid crop.
• Supplier recommended release rates or guidelines vary depending upon susceptibility of crops, length of time, and infestation levels and may range from from 50 million nematodes (1 pack) per 800 or 1700 square feet
• Use potato slices to monitor for fungus gnat larvae. (More slices work better than less). Place slices on pots two days before application (to determine levels before treatment), 3 to 5 days after application and 10 to 12 days after application. In each case, wait 2 days before checking the slices for the fungus gnat larvae. 
• The symbiotic bacteria breaks down the host insect’s cuticle. The infected larvae rapidly disappears, so may be difficult to locate. Infected fungus gnat larvae are often opaque-white to light yellow in color.

For more information on Steinermena feltiae:

NemaShield – BioWorks www.bioworksinc.com (accessed 9/07)

Nemasys – BeckerUnderwood - www.beckerunderwood.com (accessed 9/07)

ScanMask – IPM Laboratories – www.ipmlabs.com (accessed 9/07)

Entonem – Koppert - www.koppert.com (accessed 9/07)

Searchable Database On Insect Parasitic Nematodes: www.oardc.ohio-state.edu/nematodes (accessed 9/07)

 

Predatory Mites

A native, soil-dwelling predatory mite, Stratiolaelaps scimitus (sold under the name of Hypoaspis miles) feeds on fungus gnat larvae. This mite especially prefers small, first instar fungus gnat larvae.  Hypoaspis also feeds on other soil dwelling insects such as springtails and root mealybugs.  Larvae, nymphs and adult Hypoaspis are predatory. When small prey insects are scarce, H. miles can survive by scavenging on plant debris and algae. This mite inhabits the top ½ inch of soil and does not survive in standing water. Hypoaspis feeds upon thrips pupae and prepupae found in the soil, too. However, it cannot be relied upon exclusively for thrips control and is most effective when used with other thrips biological control agents. Hypoaspis aculifer is another naturally occurring predatory mite that also feeds upon bulb mites as well as fungus gnat larvae. But, it has not been as well studied as H. miles.  

Hypoaspis is shipped in a vermiculite/peat carrier with all stages of the predatory mites in 1 quart containers with a shaker lid. This mixture may also contain mold mites that are a food source for Hypoaspis. This mixture can be distributed over the media surface, especially when pots are placed close together. It is not necessary to apply the mites to every flat of bedding plants, as they may move about. However, these predatory mites won’t move through an entire greenhouse from a single introduction point.  

Hypoaspis life cycle from egg to adult is about 18 days at 68°F. They do not go into diapause in the winter. Each mite consumes from about 1 to 5 prey per day. Hypoaspis predatory mites are best used before fungus gnat populations become established or while numbers are still low. If fungus gnat populations are established, use with Bacillus thuriengienis subsp. israelensis.  

Tips for Use

• Use preventatively, at planting time.
• Apply as soon as possible after receiving.
• Do not refrigerate. They don’t store well. 
• Do not mix into the growing media because they will not survive. 
• Minimum media temperature is 60°F, they become inactive below 59°F
• Media should be moist, not wet.
• If you have dirt floors, it may be helpful to treat those areas also.  Treating the perimeter of the greenhouse may also help.
• To evaluate quality, place a small sample of the mites on a sheet of white paper. Hypoaspis will be tan and will move quickly. Adults are less than 1 mm. (0.039 of an inch) long.  The food source mites are translucent, white and move slowly.
• Supplier recommended release rates or guidelines vary depending upon the crop, length of time, infection pressure and whether H. miles is used alone or in conjunction with other natural enemies and ranges from 50 to 1000 mites per square meter (or ~ 10.7 square ft).
• Consult with your supplier for pesticide compatibility information or with the following resources on the Internet:

- Pesticide Side Effects Database – www.koppert.com

- Pesticide Side Effects Database - www.biobest.be/

• Hypoaspis is not compatible with chloropyrifos (Duraguard), but is compatible with pyriproxyfen (Distance), novaluron (Pedestal), fosetyl-Al (Aliette), and mefenozam (Subdue maxx) (Cabrera, Cloyd & Saborski 2004). 
• Use a 10 to 15x hand lens to look for the small (less than 1 mm (0.039 of an inch long), predatory mites in the top inch of media and at the base of the plants.

For more information:

Enotomite M- http://www.koppert.nl/cgi-bin/x031.pl?ktrn_srcID=35&lang=e

http://www.syngenta-bioline.co.uk/controldocs/html/HypoaspisMiles.htm (accessed 9/07)

Hypoaspis miles Technical Bulletin – www.biconet.com/biocontrol/infosheets/hypoaspisBulletin.html  

 

Bacillus thuringiensis subsp. israelensis (BT) 

The microbial insecticide, Bacillus thuringiensis subsp. israelensis, a soil borne bacterium, is sold under the trade name of Gnatrol™. The bacteria must be ingested by the larvae for it to be effective. A toxic protein crystal is released into the insect's gut, larvae stop feeding and die.  In a recent study (Cloyd and Dickinson 2006), it was found that Bacillus thuringiensis subsp. israelensis is more effective against the young (first instar) fungus gnat larvae than the later second and third instar larvae. The younger instars do not have to consume as much material for it to be effective. Because larger instars are less susceptible, repeated applications, i.e. two or three applications at high rates, may be needed to provide effective control. This is especially of concern with overlapping generations of fungus gnats that are common in production greenhouses.

Tips for Use

• Apply against young first instar larvae in ornamental and nursery plantings and vegetable plants
• Repeated applications needed
• Gnatrol can be applied as a thorough soil drench with conventional spray equipment or through an injector. Treat breeding areas under benches.   
• It should not be injected in combination with fertilizers or fungicides containing copper or chlorine

For more information see label at: http://www.valent.com/ (accessed 9/07)

 

Rove Beetles

Rove beetles (Atheta coriaria) are generalist predators that feed upon shoreflies, as well as fungus gnats, and thrips.  

Adults are slender, dark brown to black hairy beetles, about 1/8 of an inch long, with very short wing covers. Because the adults can fly, this helps them disperse in the greenhouse.  Larvae are cream colored to brown depending upon their age. Both stages are primarily found in the growing media, hiding in cracks and crevices. Once established in a greenhouse, they will be there year round, but population levels vary depending upon prey populations.  

The life cycle of rove beetles from egg to adult is about three weeks at 77° F. Adult females lay approximately 8 eggs per day for the first two weeks after they mature. Eggs hatch in 3 to 4 days into creamy white larvae. (There are three larval stages after which they pupate.)  

Rove beetles were first introduced as a biological control agent in  2002. More research and grower experience is needed to determine their effectiveness under New England  greenhouse conditions. Because rove beetles are a generalist predator, they may feed upon beneficial species. For example, in one study, they fed upon eggs of a predatory mite, H. aculifer as well as pest eggs. Rove beetles consumed beneficial predatory mite eggs and very young nymphs. In addition, H. aculifer mites fed upon rove beetle larvae, especially the first instar larvae.   

Rove beetles are commercially available as adults in either a tube or in a bottle with peat moss as the carrier (depending upon the amount ordered).  

For more information:

Rove Beetle Technical Sheet from Biobest -http://207.5.17.151/biobest/en/productfiches/Atheta-System.pdf (accessed 9/07) 

Tips for use

• Release upon arrival

Can be stored for a short while at 50-59° F and 85% RH

• Supplier recommended release rates or guidelines vary from 2 to 5 beetles per square meter (~ 10.7 square ft) and can be increased up to 10 beetles per square meter (~ 10.7 square ft) in hot spots.
• Check with your supplier for pesticide compatibility information or with the following resources on the Internet:

- Pesticide Side Effects Database – www.koppert.com

- Pesticide Side Effects Database - www.biobest.be/

• Compatible with S. feltiae, and diflubenzuron (Adept) (Jandricic, S, C.D. Scott-Dupree, A.B. Broadbent, C. R. Harris, and G. Murphy. 2006).
• When using potato slices to monitor for fungus gnat larvae, one may see rove beetles. Canadian researchers dipped potato slices in powdered, pelletized fish food, which is highly attractive to the rove beetles.

 

Natural Enemies Not Yet Commercially Available  

Some natural enemies of fungus gnats such as hunter flies, and synacra flies are not commercially available but may be naturally present in greenhouses.  

Hunter Flies  

Since 2002, hunter flies (Coenosia attenuata) have been found in greenhouses in New York, Maine, and Ontario chasing and feeding upon fungus gnats. The hunter fly is in the same family as a housefly but is smaller. Hunter fly larvae will feed upon fungus gnat larvae. Most of their life cycle is in the ground.  

Parasitic Wasps

Parasitic wasps (Synacra flies) are a natural enemy of fungus gnats and may be seen on yellow sticky cards. See link to image: http://www.omafra.gov.on.ca/english/crops/facts/06-079f12.jpg (accessed 7/07)

 

In summary, entomopathogenic nematodes, soil dwelling predatory mites, rove beetles and Bacillus thuringiensis, subsp. israelensis can all be part of a biological control program for fungus gnats. Regular monitoring to insure preventative use, in conjunction with cultural controls help ensure their successful use.  

Leanne Pundt
Extension Educator
University of Connecticut

September 2007 

References:

Cabrera, A. R., R. A. Cloyd, and E. R. Zaborski. 2004. Effects of Greenhouse Pesticides on the Soil Dwelling Predatory Mites Stratiolaelaps scimitus (Acari: Mesostigmata: Laelapidae) Under Laboratory Conditions. J. Econ. Entomol. 97(3): 793-799

Cabrera, A. R., R. A. Cloyd, and E. R. Zaborski. 2005. Lethal and Sub-Lethal Effects of Novaluron (Pedestal) on the Soil-Dwelling Predatory Mites, Stratiolaelaps scimitus (Womersely)(Acari: Mesostigmata: Laelapidae), under Laboratory Conditions. J. Entomol. Sci. 47-53.

Cloyd, C. 2003. Do Beneficial Nematodes Really Work? GrowerTalks. November 2003. 72-74.

Cloyd, R. 2004. Rove beetle may be an option for fungus gnat control. GMPro. Nov 2004. 74-75.

Cloyd, R. A. and A. Dickinson. 2005. Effect of Bacillus thuringiensis subsp. israelensis and neonicotinoid insecticides on the fungus gnat Bradysia sp nnr. coprophila (Lintner) Pest Management Science. 62:171-177.

Harris, M.A., Oetting, R.D. and W.A. Gardner. 1995. Use of entomopathogenic neamtodes and a new monitoring technique for control of fungus gnats, Bradysia coprophila (Diptera: Sciaridae) in floriculture. Biological control 5: 412-418

Heinz, K.M., R.G. Van Driesche, and M.P. Parella (ed). 2004. Bio Control in Protected Culture. Ball Publishing, Batavia, Ill. 522 pp. 

Hoebeke, E. R., E. Sensenbach. J. Sanderson, and S. Wraight. 2003. First Report of Coenosia attenuata Stein (Diptera: Muscidae), an Old World ‘Hunter Fly’ in North America. Proc. Entomol. Soc. Wash. 105(3):769-775.

Gaugler, R. Nematodes (Rhabditida: Steinernematidae & Heterhabditidae). Biological Control: A Guide to Natural Enemies of North America. www.nysaes.cornell.edu/ent/biocontrol/pathogens/nematodes.html (accessed 7/07)

Georgis, R., H. Kaya and R. Gaugler. 1991. Effect of steinernematid and heterorhabditid nematodes on nontarget arthropods. Environ. Entomol. 20:815-22. 

Murphy. G. 2004. The Rove Beetle Atheta coriaria in Commercial Greenhouses. Ontario Ministry of Agriculture and Food. 4 pp.

Jandricic, S, C.D. Scott-Dupree, A.B. Broadbent, C. R. Harris, and G. Murphy. 2006. Compatibility of Atheta coriaria with other biological control agents and reduced-risk insecticides used in greenhouse floriculture integrated pest management programs for fungus gnats. Canadian Entomologist 138(5):712-722.

Lopes, P. and L. Berg Stack. 2006. New England Greenhouse Floriculture Guide: A Management Guide for Insects, Diseases, Weeds and Growth Regulators. 2007-2008. New England Floriculture, Inc.

Malais, M.H. and W. J. Ravensberg. 2003. Knowing and recognizing: The biology of glasshouse pests and their natural enemies. Koppert Biological Systems and Reed Business Information. The Netherlands.  288 pp.

Sensenbach, E., J. Sanderson and S. Wraight. 2004. Hunter Flies: Good Guys in the Greenhouse. GrowerTalks. August 2004. 85-86.

Thomas, C. 2005. Greenhouse IPM with an Emphasis on Biocontrol. Publication No. AGRS-96. 89 pp. Pennsylvania Integrated Pest Management Program.