Photo Source: IPM Handbook
The major insect pest of mushrooms in North America is the sciarid fly. These flies are small black insects about 1/4 inch long with long antennae and gray wings with a characteristic split vein. Females are more abundant and larger than males. Females are attracted to lights and frequently can be seen on backlit windows, vents, picking lights and black light traps. This attraction to light provides the grower with a means to monitor the number of female flies entering the house and emerging from the compost/casing during the crop.
Sciarid larvae are translucent, white legless maggots. The head is large and dark, distinguishing sciarid larvae from other insect larvae that might be found in mushroom production houses.
Adult sciarids prefer cool temperatures and are most active when outdoor temperatures are between 50°F and 75°F. The threat of infestation is greatest from March to July and September through late November. Flies will land and lay there eggs on compost that is closest to the point of entry. It has been suggested that flies can detect residues of Dimilin and avoid laying eggs on substrate with this pesticide. They also can detect the presence of Trichoderma and may lay their eggs in areas where this fungus is growing. Eggs hatch in a few days and several larval stages occur before the adult stage is reached. Larvae prefer to fee on developing mycelium and compost.
Depending on when the eggs were laid around spawning time the first generation of flies emerges as adults just before first break. Timing of this development allows the grower to apply insect growth regulators and biological controls such as nematodes to the most susceptible life stages of the insect by timing development from the peak invasion on black light monitors. The complete life cycle requires about 28 days at normal compost temperatures.
Control methods include exclusion and chemical. It is essential to protect the crop by placing plastic on the bed surfaces and keeping the doors and other entry points closed during this period. Black light monitors are used to assess the tightness of fly exclusion measures and also pinpoint the time of invasion. Flies are capable of finding cracks in walls to find entry into rooms during Phase II and after spawning. Two main types of insecticides are used to help control the flies, insect growth regulators (IGR) and adulticides. IGR are very dependent on timing of the application. How specific the action of the chemical is to the stage of the fly larvae development will determine the timing of the application. Monitoring fly activity, specifically, when the first adult flies enter a room, may determine the timing for applying each chemical. Please consult the specific chemical’s spec sheet for this information. Adulticides are used as preventative applications that target the adult stage of the fly. These are used as a back up to exclusion and when hight adult populations occur during the cropping period. Preventative applications serve the same role as physical exclusion. Instead of making if physically impossible for fly entry, however, a chemical barrier is applied in an attempt to kill the fly before it gains access to the growing room. This is not as effective as physical exclusion, but as a backup, if some entry points have been overlooked. Preventative applications do not replace diligently sealing the growing room. On the contrary, the two must work together.
Only use spray when weather is conducive for fly movement and when significant fly populations exist on the farm. Every farm will be different, and each farm should develop its own procedures dictating when spraying should take place. If the temperature is below freezing outside and your growing rooms are physically separated, flies cannot move from old to new rooms or from the wild population into new rooms. Determine at what temperatures they will move on your farm, and do not use preventative sprays when the outside temperature is below the established figure.