FACTSHEEl IJ (I& PESTS

® Ontario Ministry of Agriculture,Food and Rural Affairs

ORDER NO. 06-079

AUGUST 2006

290/621

FUNGUS GNATS AND SHOREFLIES INGREENHOUSE CROPS

G. Ferguson, G. Murphy, L. Shipp

INTRODUCTIONFungus gnats and shoreflies are small black flies oftenobserved in the vicinity of the growing media ofgreenhouse crops. These flies, often regarded as nuisancepests, can facilitate and transmit root diseases in allgreenhouse vegetable and ornamental crops and reducethe aesthetic quality of ornamental crops.

DESCRIPTION AND LIFE HISTORYAdult fungus gnats are grey to black, about 3-4 mmlong with long legs, thread-like antennae, and largecompound eyes that meet above the base of theantennae. Adults resemble small mosquitoes (Figure 1)and are weak fliers frequently observed resting on themedia surface. Each female lives about 10 days and laysabout 150 oval, white eggs in the organic matter of themedia. Eggs hatch in 2-7 days, depending on thetemperature, producing white larvae that are 4-6 mmlong. The larvae have 12 abdominal segments and adistinctive shiny black head (Figure 2). The larvae feedfqr 5-14 days before pupating, after which adultsemerge in 4-6 days. As with other insects, fungus gnatsbecome more active and reproduce more quickly atwarmer temperatures. The life cycle can be completed in21 days at 24°C, compared with 38 days at 16°C.

Adult shoreflies are similar in size to fungus gnats butresemble small houseflies, having a dark chunky bodywith short, bristle-like antennae and short legs(Figure 3). They are stronger fliers than fungus gnats,and have five clear spots on the wings. Adult shorefliesprefer wetter conditions than fungus gnats and are oftenfound in wet areas under benches and other water­soaked areas. Females lay eggs on algae or moist growingmedia and larvae hatch in 2-3 days. Shorefly larvae arecream to muddy brown in colour (Figure 4), appearheadless, and feed on algae and other micro-organismsfound in the media for 3-6 days before pupating. After afurther 4-5 days, the adults emerge. Shoreflies takeapproximately 9-14 days to complete a generation at

greenhouse temperatures. Generally, adults and larvaefeed on algae. Howev;er, larvae may feed on rootsinfected with fungi.

DAMAGEThe immature stages of fungus gnats generally feed ondecaying organic matter, soil fungi and algae, and cancause direct damage by feeding on fine roots, root hairsand on tender lower stems. All greenhouse crops can beaffected. They also cause indirect damage by creatingentry points in the roots for disease-causing organisms.Fungus gnats can themselves transmit such disease­causing organisms. For instance, hardy Pythium sporesingested by the larvae can remain in the gut until thelarvae have developed into the adult flying stage. Variousstudies indicate that fungus gnat adults can spread sporesof disease-causing fungi such as Pythium and Rhizoctonia(Figure 5) by flying to non-infected plants and excretingthe spores. Adult flies can also spread Fusarium,Verticillium and other fungi by means of spores that arecaught on their legs and bodies. Organic media such aspeat and cocofibre favour reproduction of fungus gnats.

Immature shoreflies, by contrast, are semi-aquatic, feedprincipally on algae, and do not normally feed on plantparts. However, they will feed on roots infected withfungi and can therefore spread disease if ingested sporesremain viable in the gut until the adult stage is reached.Research indicates that shoreflies can acquire and spreadPythium spores in a manner similar to that of fungusgnats. Shoreflies also cause cosmetic damage toornamental crops by depositing black drops ofexcrement on leaves and flowers.

Bringing the Resources of the World to Rural Ontario

FIGURE 3. Shorefly adult. Note very short antennae,stout body and clear spots on wings.

FIGURE 4. Shorefly larva.

FIGURE 1. Adult fungus gnat on sticky card.Note long legs and antennae.

FIGURE 2. Fungus gnat larva.

FIGURE 5. Fungus gnats can transmit diseasessuch as Rhizoctonia shown on this poinsettia.

FIGURE 6. Use of potato slices can be very effectivein monitoring for fungus gnat larvae and the predatoryrove beetle.

MANAGEMENT STRATEGIESGenerally, control of rhese flies can be difficult becauseof the above ground stage, continuous overlappinggenerations and short life cycles. To avoid problemsassociated with these flies, implement control measuresearly. Strategies for management of these flies includemonitoring and cultural controls. Whereas biologicalcontrol agents (BCAs) can be effecrively used forsuppression of fungus gnars, the effectiveness of many ofthem against shoreflies is uncertain.

MonitoringYellow sticky cards at the normal position at the top ofthe canopy will indicate the presence of these flies. Butfor earlier detection and greater trapping effectiveness,such cards are best placed horizontally at the base of the

plants. For indication of levels of fungus gnat larvae,place slices of raw potato on the media and examine after2.4 hours with a magnifying lens (Figure 6).

FIGURE 7. Control of algae on rockwool (7a) andunder benches (7b) is important in the management offungus gnats and shoreflies.

General Control Strategiesit is best to prevent establishment of these £lies in thegreenhouse by implementing good hygiene practices,and having good drainage to eliminate puddles andformation of algae (Figure la, b). Minimizing exposureof media surfaces will block light and thus help preventgrowth of algae.

Biological ControlSeveral commercially available Biological Control Agents(BCAs) can be used for managing fungus gnatpopulations. These include a bacterial insecticide,Bacillus thuringiensis subsp. israelensis; a parasiticnematode, Steinernema feltiae; a predatory mite,Hypoaspis spp.; and a predatory beetle, Atheta coriaria,commonly called the rove beetle. The effectiveness ofthese agents has been determined mainly for fungus gnatlarvae and may not be equally effective against shore£lylarvae. There are also several naturally occurring BCAsthat are not available commercially, but are often foundin greenhouses where pesticide use has been reduced.

FIGURE 8. Steinernema feltiae nematodes canprovide effective control of fungus gnats.

Hypoaspis: This predator is a soil-dwelling mite (Figure 9)that feeds mainly on the young larvae of the fungusgnat, very little on eggs, and probably not at all onpupae. They do not diapause but will become inactiveif media temperatures fall below 15°C. If thesepredarors are released in the seedling stage, make asecond release after planting out in the main housewhen there is increased likelihood of available food rosustain them. It is best to release these predatOrs befotefungus gnat populations are established. Theadvantage of using this predator is that it providesseason long suppression, and will also feed on othersmall insects in the growing media, such as springtails,and thrips pupae.

FIGURE 9. Hypoaspis predatory mites feed on soil­dwelling organisms such as fungus gnat larvae, andother pests such as thrips pupae.

Coenosia attenuata is a predarory fly that attacks bothfungus gnats and shoreflies, Synacra and HexacoLaneoscateLLae are parasitic wasps that attack fungus gnatsand shoreflies respectively. Some notes about use ofthese BCAs follow.

Bacillus thuringiensis subsp. israelensis (Bti): Bti consistsof two kinds of spores, one is active and the other is astorage spore that includes a roxic protein crystal.Once these spores are ingested by the larva, thealkaline pH in the gut facilitates release of the roxiccrystal. This crystal destroys the gut wall of theinsect, allowing the active spotes to pass into theblood stream. The insect then dies from bloodpoisoning. Within 24 hours of ingesting Bti, fungusgnat larvae stop feeding and become limp. Deathfollows 1-7 days after ingestion. This bacterium doesnot kill by contact and only the larval stage is killedby ingesting Bti. Studies show that the younger larvalstages of fungus gnats are more susceptible andtherefore multiple applications must be applied toachieve control. Adults do not feed on these sporesand are therefore unaffected. When using thisbacterium, ensure that the pH of the water used formixing is neutral or slightly acidic (upper limit is pH= 7.0). Alkaline water, or the addition of any othersubstance that will raise the pH, will render thisorganism ineffective.

Steinernema feltiae: These nemarodes may provide morerapid control of fungus gnat larvae than the otherBCAs. Pupae are not as susceptible to the nemarodesas the larval stages. The nemarodes search out aninsect host and enter it though body openings such asthe mouth, anus, and breathing pores called spiracles.Once inside the insect, the nemarodes release abacterium (Xenorhabdus spp.) which they carry in theirgur. The bacteria develop within the insect, killing itwithin 48 hr. The nemarodes can then theoreticallydevelop by feeding within the larva (Figure 8).However, the size of fungus gnat larvae may be roosmall to accommodate their reproduction. For bestresults, apply at the end of the day to avoid dlying outand exposure ro direct sunlighr. Applying them at tl1istime also reduces the risk of flushing the nemarodesout of the growing media by irrigation of the fertilizersolution over the course of the day, particularly inrod.'Wool. Studies show that these nematodes movevelY easily through rockwool, and that within twoweek~ after application the majority of nemarodes arefound in the lower parts of the rockwool media.Nemarodes are most effective when the temperatureand pH of the water used for mixing are the same asthose required for optimum crop growth. Applicationsshould be made weekly for several weeks, dependingon pest populations.

Atheta coriaria: This beetle is a relative newcomer to thebiological control arsenal for fungus gnats.Laboratory studies indicate that it holds muchpotential as a control agent for fungus gnats andshoreflies. The adult is a small black beetle, 3-4 mmlong (Figure 10). There are three larval stages thatvalY from white in the earlier stages to yellow-brownin the final stage. The adult and all larval stages arepredatory. The rove beetle is very active, establisheseasily and spreads quickly throughout thegreenhouse. It often establishes naturally and residentpopulations will maintain a permanent presence.Potato slices used to monitor fungus gnat larvae arealso vety effective in detecting the presence of Athetaadults and larvae.

FIGURE 10. The rove beetle Atheta coriaria is a soil­dwelling predator that feeds on the eggs and larvae offungus gnats and shoreflies.

Coenosia attenuata (also called the hunter fly or tiger fly)is a greyish, predatory fly in the same family as thehousefly (Figure 11). It is larger than the shorefly andthe adult preys on other flying insects, catching themin flight. It feeds on fungus gnats and shoreflies, butalso on other flying insects such as leafminer and, toa lesser extem, whiteflies. Coenosia larvae live in theoil and are generalist predators on other soil­

dwelling organisms such as fungus gnat and shoreflylarvae.

FIGURE 11. Coenosia attenuata is a predatory fly, theadult of which feeds on flying insects such as fungusgnats and shoreflies, and whose larva feeds on soil­dwelling organisms. The figure shows the sizecomparison between a shorefly on the left andCoenosia on the right.

Synacra is a parasitic wasp that lays its eggs into fungusgnat larvae. The wasps develop and emerge from thefungus gnat pupae. The adult wasp is similar in sizeto the fungus gnat but has a typical wasp-likeappearance wirh a pinched "waist" and a long,tapered abdomen (Figure 12). Although itseffectiveness in controlling fungus gnat populationshas not been documemed, large numbers of theseBCAs are often found on sticky cards in greenhouses.

FIGURE 12. Synacra is a parasitic wasp that lays itseggs into fungus gnat larvae.

Hexacofa neoscatelfae is a parasitic wasp that feeds onshoreflies (Figure 13). It is smaller than a shorefly andcan be found in large numbers on yellow sricky cardsin greenhouses with residem shorefly populations. Itis black in colour and has an almost sphericalabdomen, in comrast with the more elongatedabdomen of Synacra (Figure 14).

FIGURE 13. Hexacola neoscatellae is a parasiticwasp that lays its eggs into shorefly larvae.

FIGURE 14. Comparison in size between Synacra(left) and Hexacola. (right).

Chemical ControlUse of pesticides [Q control fungus gnats is targeted

. against the larval life stages living in the growingmedium. Best results are usually obtained when appliedearly in the crop production cycle, which is when fungusgnat populations are often at the worst. As the cropmatures, the growing medium dries our more quicklyand the developing root system is less prone [Q feedingdamage. There are several pesticides registered forcontrol of fungus gnats and shoreflies, some of which arecompatible with biological control programs. Please refer[Q OMAFRA Publication 370, ProductionRecommendations fOr Greenhouse FLoricuLture, andPublication 371, Growing Greenhouse VegetabLes.

This Factsheet was written by Graeme Murphy,Greenhouse Floriculture jPM Specialist, OMAFRA, VinelandStation; Gillian Ferguson, Greenhouse Vegetable IPMSpecialist, OMAFRA, Harrow and Les Shipp, ResearchScientist, Agriculture and Agri-Food Canada, Harrow.