Megaselia halterata

The mushroom phorid fly (Megaselia halterata) is a species of scuttle fly or hump-backed flies in the family Phoridae. "The mushroom phorid" is also used to refer to M. halterata. Megaselia halterata is a common pest of mushroom cultivation, attracted by the aroma of developing fungal mycelium. The larvae damage both the mushroom mycelium and gill tissues. Megaslia halterata can be found worldwide. Megaselia halterata is an important pest of mushroom cultivation, capable of severely limiting mushroom yield. Megaselia are naturally infected by Howardula nematodes, however entomopathogenic nematodes do not readily control M. halterata.

from Wikipedia