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Dealing with Mushroom Flies

‘Mushroom Flies’ Damn!

‘Mushroom Flies’

Damn! some bugger fly just flew out of my terrarium! wtf?yaaaarrrgghhh! is my crop ruined?

Atsome time or other, all mycologists will encounter 'fungus gnats', thepesky flies that are real pests, and need action. these gnats are veryclosely associated with fungi and decaying vegetation and highlyattracted to mycelium and mushrooms. (In fact, I often wonder whether Iam really a fungus gnat myself.) They lay eggs in the compost or casinglayer, which hatch into critters that, like the adult flies, nibble andtunnel through mycelium, growing and marture mushroooms. Flies flitfrom tub to tub, often in swarms like locusts on a wheatfield. They mayalso be the primary vectors for spreading fungal, bacterial and viraldiseases, mites and nematodes. Also, the wounds created by larvalfeeding provide an entry site for secondary soil-borne pathogens.

can you spot him in this picture? what about up close?

Uncontrolledpopulations of any of the main mushroom pests can result in substantiallosses in yield due to both direct larval action and associated diseasespread by adult flies. Problems caused by flies found in croppinghouses include reduced yield by damaging the compost and feeding on themycelium, acting as vectors of diseases, a nuisance to thegrowers/pickers, and possible crop rejection at market. who would goback to again buy a packet of mushrooms that had flies flitting out ofit?

Mushroom flies fit into several categories, only one of which(sciarids) represents the true ‘fungus gnat’.[Wherever I refer to ‘fungus gnats’ in the rest of this document, thesame will largely apply also to phorids, cecids and sphaerocids.]

Likeall flies, fungus gnats have four developmental stages: egg, larva,pupa and adult. The female adult flies deposit between 50 to 300 eggs(0.2mm) in the compost and the developing larvae pass through fourmoults (varying in size from 1 to 8 mm) before pupating.

Outbreaksof high fungus gnat populations are linked to high humidity and soilmoisture levels - this may have something to do with the insect'svulnerable egg stage. Fungus gnat eggs are very much subject todesiccation.

When the moist conditions that favour eggdevelopment are present in a species that goes from egg to adult inonly 10 to 14 days, populations are likely to boom. High humidity andsoil moisture encourage the growth of the larval stage.

If allowed to enter and uncontrolled, fungus gnat numbers can sometimessoar into an explosion of winged insects.


Withoutclose examination, these small flies appear to be very similar and arehard to distinguish from each other. However, it is important to knowwhich pest you are dealing with, as treatments are often very speciesspecific - different species of fly might have different susceptibilityto munchers and to insecticides so is also important to know whichspecies is present on a crop.

1) True Fungivorids - Sciarid Flies:
Lycoriellacastanescens (auripila) and Lycoriella ingenua (mali); Bradysiadifformis (paupera), Bradysia coprophila, and Bradysia lutaria.

Adultflies are usually 3 - 4 mm long and with the naked eye can bedistinguished from the other fly pests of mushrooms by their longantennae, black shiny heads, long legs and long thin wings. They looklike bonsai mosquitoes.

Sciaridsare compost feeders and really prefer unspawned compost to thatcolonised by Agaricus mycelium. The adult flies are attracted byvolatiles given off by the compost so are a danger at any time afterpasteurisation when the compost is cool. They feed on rottingvegetation, so compost is an ideal substrate for them.

Althoughexudates produced by the growing mycelium inhibit the larvae, they havebeen observed browsing on the growing tips and when there is a largeinfestation the larvae will even burrow right up into the mushroomstipes, the little fuckers. The accumulation of their waste (calledfrass) also renders the area unsuitable for mushroom growth and makesthe place generally unsavoury.

2) Phorid flies:
Megaselia species eg. Megaselia halterata

Thephorids, also known as humpbacked flies, resemble fruit flies inappearance but do not have the red eye classic trademark of the fruitfly. Phorid flies are larger than sciarids, with very short antennaeand a characteristic hump-back.

Phoridlarvae are obligate mycelial feeders therefore the adult flies are notattracted until after spawning. They are unable to fly when thetemperature falls below 12C and are therefore unlikely to re-infestmushroom houses between late Autumn and early Summer. The larvae arewhite, 1 to 6 mm long, are stubby at one end and have a pointed head atthe other. They feed on the growing mushroom mycelium but rarely feedon the fruiting body itself, although some species are known to do so.They can be distinguished from sciarid larvae by the absence of theblack head and they develop more rapidly into a pupa.

Thetrademark of the adult Phorids is running, betraying their evil traitof cowardice: even with the naked eye you can watch adult Phorid fliesdashing rapidly across surfaces with a rapid, jerky, running movementinstead of immediately flying when disturbed.

3) Sphaerocid flies
eg. Pullimosina heteroneura

Theseare an occasional pest but are usually an indication of poor compostpreparation. These small flies have dark colored bodies and are about3-4mm in length, making proper identification difficult withoutmagnification: the tarsi (last 5 segments of the hind leg) are the keyto identifying the Sphaerocerid fly - the first segment is greatlyenlarged in sphaerocids.

Mostflies of this family generally breed in animal manure and can also beknown as small dung flies. There are a few species that feed in anydecaying organic matter, meaning that they can be found in mycologists’compost.

4) Cecids:
eg. Heteropeza pygmaea and Mycophyla speyeri

Cecidsare only usually seen as larvae. Cecid flies are rarely seen as thelarvae reproduce paedogenetically - i.e. new generations are producedwithin the body of a 'mother' larva without sexual reproduction - andthe flies are very small.

Thelarvae are obligate mycelial feeders as with the phorids. In a mushroomcultivation environment i.e. high temperature and humidity, cecids canproduce a new generation every 4 to 7 days and each 'mother' larva canproduce up to 12 new larvae so numbers can increase exponentially. Whenthere is a bad infestation the larvae clump together and (get this)flow over the sides of the mushroom beds onto the floor where they canbe inadvertently transported to healthy crops on feet and equipment.Ewwwww.

Cecids are less common nowadays as hygiene and carefulselection of casing materials eliminate them as a problem. However, aswith the phorids, there is no pesticide available for use against them.

5) Not actually mushroom flies : Thrips, Fruit Flies
Although many growers also refer to their infestations as ‘thrips’ or ‘fruit flies’,this is a misnomer - neither of these are actually mushroom flies :thrips lust after not fungus but flowers and leaves, and fruit flies(similar to phorids, but with red eyes) prefer …. (can you guess?)..fruit!.

...fruit fly

Perhaps in time of famine, these may pay a visit to a mushroom growingaea, but mostly these have other fish to fry.


Thebest control for flies is strict sanitation, exclusion and farmcleanliness. Stop the buggers ever getting a sniff. Mushroom housesmust be airtight and all air vents must have filters. If you can affordto make your growing area a positive pressure zone, with filters on allair intakes, you'll be doing youtself a major favour in pest prevention.

Watermanagement is crucial to controlling fungus gnat buildups: avoidoverwatering, and provide adequate indoor ventilation. Moving air isnot good news to fungus twats ... er gnats.


dried coffee grounds, tobacco water (from soaking cigarettebutts), chilli spray and garlic water have all also been toutedas possible deterrents. citronella candlesare reputed to deter many flying insects, including fungus gnats. also,crushed leaves of the herb 'tansy'can be sprinkled around the growing area: In Europe and in colonialAmerica in the 1840s, meat was packed in tansy or rubbed with it toprevent decay and to repel flies. The oil distilled from the plant,made a 'mosquito dope' useful to hunters and fisherman and others whohad to work where mosquitoes are troublesome.

One major sourceof gnats is in their (microscopic) egg and larvae forms, in coco-coirand under-pasteurised manure-based substrates. Increase pasteurisationtimes and temperatures (being wary not to accidentally sterilise).Remove any standing water, or treat it with the long-term biologicalmosquito larvicide Bacillus Thuringiensis (variants H-14[Gnatrol] or Israelensis, BTI)weekly for two or three weeks as routine preventative control of fungusgnat larvae. This larvicide gradually settles in water where it iseaten by any mosquito or fungus gnat larvae growing there, which willbe killed continuously.

BTI is used to make ‘skeeter dunks’ thatlook like little donuts (designed to float on water and killmosquitos), and will keep on working for 30 days or longer undertypical environmental conditions. While floating they slowly release atthe waters surface. Put some in water to be used for a day or two, toload it with the stuff.


You can also drench soil with neemextract. the fascinating Indian Neem Tree (Azadirachta indica) and itsunique extracts have an enormously broad range of applications. Theoil, extracted from neem tree kernels, has nourishing qualities and isused in India today in many toiletry ranges, including hair-careproducts, toothpaste and soap. Mahatma Gandhi believed firmly in thegoodness of neem and ate neem leaf chutney as part of his everydaydiet. The main substance azadirachtin, a tetranortriterpenoid,influences the hormonal system of insects, exerting thereby apesticidal effect. Feeding activity, reproduction and flying ability ofinsects are also affected. Azadirachtin has a very low toxicity tomammals, its biologically degradable and can be easily extracted fromthe seeds of the trees - it can be found as leaves, oil, powder orcoir. Note, though that Azadirachtin [Diazinon] has been withdrawn dueto fly resistance from many areas.



"Youget more with honey than vinegar" is suggested by some as ways to temptthe flies from an already-infected casing to their place of execution,to be swatted, stuck, drowned, electrocuted, poisoned, eaten orotherwise assassinated.

Others see this as folly, fearful ofbringing more baddies in by attracting them from outside, where theywere previously not troublesome. This is one you'll have to decide foryourself.

dishes of vinegar, bourbon, beer,watered dish soap, ‘minty mouthwash’ have all been touted hereat the shroomery. Most entertaining of all is the ingenious andold-fashioned 'fly agaric'in a dish of milk, the first to get em stoned and the second to drownthem. This inventive use is behind this nickname of the amanitamuscaria. It has been used as a fly killer - hence the name. Smallpieces of the fungi were added to a saucer of milk. Flies came to feedfrom the saucer and were killed.

More recommended are those yellow (they love the colour) stickypheromone-scented flypaper cardsnear lights to trap the first few arriving critters for monitoringpurposes or to minimise numbers.

'fly agaric':-


Starting low-tek, there's manual removal with clean tweezers,applying the business end of a vacuum cleaner hose,drowing the caing inhabitants with a 24h water dunk or a 6h bleachwater (1:200) dunk.

Also heavily supported by myco-enthusiasts are the microscopicexoskeletons of diatoms, sea-dwelling microbes. You can use diatomaceus earthat 3-5% volume in casing mixture or even in compost: it acts likeground glass in grinding their little maggoty bodies up from theinside. Ouch! Good!

And don't forget electric bug zappers,which both lure and kill in one snap.

'DE', Diatomaceous Earth:-


BIOLOGICALSmeans employing living nature to do its circle-of-life thing and workfor you. This can be as passive as a plant, a loose frog or mantis, oras aggressive as sending mites, worms or wasps to eat their eggs anddeveloping children. Venus flytrapswill survive in a humid growroom, look teriffic, and should eat thecritters (though some worry that the fine hairs may not be triggered bythe even finer gnat legs). Green treefrogswill be grateful to eat flies all day, if you can be sure they're notharbouring any unwelcome pests and microscopics of their own. Still,they're good enough for Fungi Perfecti, who made one an 'employee ofthe month'!

Praying mantises(tinodera sinesis) may sound like a scary option, but i'm not talkingabout the 4-inch scaryass-looking adults, but the babies, freshlyhatched from egg cases. I suggest you move 'em on out by the time theyget as big as your hand.

Professional mushroom house kepers release predatorymites (Hypoaspis miles)and nematodes (see below) to control fungus gnat larvae. When fungusgnat food resources are exhausted, Hypoaspis mites will turn to feed onthe nematodes -- and be present when fungus gnat populations riseagain. Hypoaspis mites are used at a quantity of approx 10/sq foot forprevention, 30-50/sq foot for damage control.

Predatory nematodes - Steinernema feltiae-(the more common species Steinernema carpocapsae offers lessimpressive results) are naturally occurring tiny worms which live inthe water-coated spaces between soil particles. These nematodes have aspecialized third juvenile stage, the dauer larva, or infectivejuvenile, which is the stage which attacks insects: it is nonfeeding,and thus can survive in the soil for extended periods until it is ableto find a susceptible host by orienting to carbon dioxide, and hostexcretory products.

Infectivejuveniles enter hosts through natural openings, such as the mouth, anusor breathing pores (spiracles). These nematodes carry specific speciesof bacteria in their intestines. Upon entering the host, the nematodereleases the bacteria, where they rapidly multiply, killing the hostthrough release of protein-destroying enzymes, usually within 24 hours.Nematodes then feed on the host remains, and complete two or threegenerations inside the host. Sounds horrible, but good on em - theseare OUR ninja assassins.

When the host resources are gone,large numbers of infective juvenile nematodes leave the host and beginto search for new hosts. At room temperature, it takes steinernematidnematodes about 7-10 days from infection to the emergence of newinfective nematodes. Insects infected by steinernematid nematodes arelimp, and cream to dark brown in color.

The combination of miteand nematode seems to work very well, but can't always be counted onfor the long run, because once they've consumed their pest hosts,they'll die or depart, leaving the plant unprotected.

Another biological control agent is the naturally occurring parasitic waspSynacra pauperi. This parasitoid is very noticeable on yellow stickycards. In fact, they are more attracted to yellow sticky cards thanadult fungus gnats. The adults are approximately the same size asfungus gnat adults (1/8 inch long) and have a noticeable constrictionbetween the head and the thorax, and the thorax and the abdomen, whichtapers to a sharp tip. Antennae are elbowed and the tips are dark andswollen. Females are reddish-brown with black eyes. Males are blackwith long antennae approximately the same length as the body.

Femalesynacra wasps insert eggs into fungus gnat larvae. They can develop ineach of the three larval instars. The parasitized larvae live untilpupation, then die, after which the wasp pupates. However, theparasitized fungus gnat larvae may still cause damage during theinterval between parasitization and death. Synacra wasps have beendemonstrated to be a useful biological control agent of fungus gnats inSwedish greenhouses to the point that insecticides are generally notneeded.

A rule of thumb with biologicals is you can't waituntil you get an infestation. All these organisms seem to work best aspreventive measures before pest levels build up, and when pestpopulations are lower. Many commercial growers just monitor with yellowstrips, and you can wait until monitoring indicates it's time to act.

CHEMICALS i.e. insecticides maybe necessary when the sticky panels or direct evidence indicatesignificant fly populations in the house:

Those used in commercial mushroom houses include
Methoprene to casing and spawn,
Diflubenzuron[Dimilin, used in aquaculture, possibly found in aquarium shops] tocomposts and casing layers. Diflubenzuron is an insect growth regulator(IGR) which inhibits the production of chitin in the larvae thuspreventing their moulting process. Over the past few years, cases ofinsect resistance to diflubenzuron have appeared.
Azadirachtin [Diazinon] tomushroom house walls and floors (withdrawn due to fly resistance frommany areas, and
Permethrin, Piperonyl butoxide and Pyrethrinsto fog mushroom houses for flies (these latter three are not used whenmushrooms are present).


prevention is better than cure
give pasteurising manure an extra half hour, and stick to 170srather than lower temps
consider getting a bug zapper, and some yellow flypaper stripshung vertically near the trays.
get hold of, and use, some diatomaceus earth (also detersslugs and snails in outdoor beds)
remove all infected casings from the grow area to minimisecross-contamination, and clean up well after yourself.
consider a permethrin-based insecticide in compost and casinglayers.
if you have a big operation, consider getting some bugs towork for you as gnat assassins

hope this was useful!

shirley xxx

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