Streams and Rivers are less likely to be contaminated with pesticide residues today than in years past, and this means more black fly larvae survive to become adults. Some years 1991 for example adult fly emergences were low and damages minimal. The trend, however, has been for ever larger emergences since the mid-1970s. The gnat season, when the adult flies suck blood from livestock, generally lasts from November through April. Black flies spend most of their lives in running water.
Adults emerge from submerged pupae hungry, swarming and feeding on blood of host animals.