Phorid flies, Megasellia halterata (Above), in general may be more numerous than sciarids, but phorid flies cause less mushroom damage than sciarid flies. However, the total economic effect of phorid feeding on mycelium is not really known.
Adult phorids are usually most common in late summer and early fall cropping. If present, the flies are attracted to light, and swarm near windows and doors in growing rooms and packing sheds. It is a common sight to see thousands of dead phorid flies on the floor beneath windows and near doors. Mushroom workers notice adult phorid flies during the day because of their daily flight schedules. Mating swarms of phorids are commonly found outside mushroom houses; once mated, the females are attracted to mushroom houses by the odor of actively growing mycelium (spawned compost). Unspawned compost and fully grown mature composts are not so attractive to ovipositing (egg-laying) female phorid flies.
Adult female phorids are about 3 mm in length. They lay smooth elongated ovoid eggs beneath the surface of compost or in the casing. The eggs hatch in 2 to 3 days, and newly emerged larvae are nearly transparent and taper towards the "head end. With maturity, they become whitish and are about 4.5 mm in length; end-to-end, about five and one-half larvae equal an inch. At 65˚F, the larvae cycle will be completed in about 14 days. The pupal stage follows; the pupae appears to be whitish initially, but quickly becomes dark yellowish-brown. The pupal stage requires 20 days at 65˚F. Adult flies may live for about 8 days, but the length of a generation varies considerably with temperature.
Wuest, P., and G. D. Bengtson. 1982. Penn State Handbook for commercial mushroom growers. The Pennsylvania State University, University Park.