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The major insect pests of mushroom cultivation are the sciarid (Lycoriella spp.) and phorid flies (Megaselia spp.). Both flies are attracted to fungi and decaying vegetation. They lay eggs in the compost or casing layer and the larvae may tunnel through mushrooms rendering them unmarketable. Flies may also be the primary vectors for spreading fungal, bacterial and viral diseases, mites and nematodes. The best control for flies is strict sanitation, exclusion and farm cleanliness. Mushroom houses must be airtight and all air vents must have filters. Fly populations are monitored using sticky panels near black lights. Chemical treatments are only applied when the sticky panels indicate significant fly populations in the house.
Non-chemical: Good sanitation and filters on all air vents in the mushroom house are used to prevent flies from contaminating the trays. Beneficial nematodes applied to the casing layer have been useful in infecting fly larvae.
Chemical: Structural or premise sprays may be used for external building surfaces, corridors, etc. Due to re-registration efforts some have been discontinued. The use of Baygon may be restricted or discontinued in coming months.
Azadirachtin - Label has a rate of 0.02- 0.04 lb ai/acre and 4 hour REI (3). In 1997, 10 lbs azadirachtin were applied to 8% of California?s mushroom acreage an average of 3 times at a median rate of 0.04 lbs/acre (1).
Diazinon - Label has a rate of 2-2.5 lb ai/50 gallons of water and 24 hour REI. Diazinon is applied to mushroom house walls and floors (3). In 1997, 1980 lbs diazinon were applied to 87% of California?s mushroom acreage an average of 6 times at a median rate of 22.3 lbs/acre (1).
Diflubenzuron - Label has a rate of 0.6-1.0 lb ai/1000 ft2 (26.1 - 43.6 lb ai/acre) of compost and 0.05 lb ai/1000 ft2 (2.2 lb ai/acre) of casing and 12 hour REI (3). In 1997, 86 lbs diflubenzuron were applied to 15% of California?s mushroom structures an average of 3 times at a median rate of 12.4 lbs/acre. An additional 617 lbs diflubenzuron were also applied to 9% of California?s mushroom acreage an average of 1 time at a median rate of 4.6 lbs/acre (1).
Methoprene - Label has a rate of 0.21 lb ai/1000 ft2 (9.1 lb ai/acre) for compost and 0.08 lbs ai/1000ft2 (3.5 lb ai/acre) for casing and spawn. The REI is 4 hours (3). In 1997, 111 lbs methoprene were applied to 0.2% of California?s mushroom acreage an average of 1-2 times per crop.
Permethrin - Label has a rate of 0.05 - 0.0625 lb ai/8000 ft2 (0.27 - 0.34 lb ai/acre), 12 hour REI, and 3 day PHI. 8000ft2 is the size of a standard double house. Permethrin is used to fog houses for flies and must not be used once mushroom pins are present (3). In 1997, 19 lbs permethrin were applied to 62% of California?s mushroom structures an average of 3 times at a median rate of 0.02 lbs ai/acre. An additional 604 lbs permethrin were also applied to 31% of California?s mushroom acreage an average of 3 times at a median rate of 0.4 lbs/acre (1).
Piperonyl butoxide and Pyrethrins - Label has a rate of 0.005-0.01 lb ai piperonyl butoxide /1000 ft2 (0.21-0.42 lb ai/acre) and 0.004-0.008 lb ai pyrethrins/1000 ft2 (0.17-0.34 lb ai/acre) and 12 hour REI. Piperonyl butoxide and pyrethrins are used to fog mushroom houses for flies and are not used when mushrooms are present (3). Piperonyl butoxide and pyrethrins were applied to 6% of California?s mushroom acreage an average of 3 times at a median rate of 0.06 lb piperonyl butoxide and 0.01 lb pyrethrins. A total of 25 lbs piperonyl butoxide and 2 lbs pyrethrins was applied to California mushrooms in 1997 (1).
You could also try those insect repellants that use sound as a control method, such as SonicWeb, or Citranella candles. These were developed for bighting insects though and might not have any effect on the flies you have. You could also try cold shocking or dunking...the mycelium would survive but the flies probably wouldn't. I have also heard that the introduction of certain nematodes can work as they eat the flies. Not sure about the methodology for this though.