In the last two winters in the Charlottesville area, we have found large numbers of Black Fly larvae in many of our streams, especially in streams we would normally consider very healthy. Black Fly larvae form colonies on rocks, and there can be hundreds (thousands?) of larvae on a single rock (see enclosed photo). Since Black Fly larvae are generally considered very tolerant of stream impairment ("6" on a scale of 1-10), large numbers in a monitoring sample can drag down the score of the stream and seem to indicate a decline in the health of the stream.
However, professional entomologists are now telling us that not all Black Flies are equally tolerant. In particular the genus Prosimulium has a tolerance value of 2-3. And, when I checked the larvae in our streams in February and March of this year, they were all Prosimulium! (Prosimulium larvae have a dark tip at the end of their antennae (see photo above) making them fairly easy to distinguish using a microscope.) The presence of this type of Black Fly in a stream might well indicate a healthy stream, therefore, not an unhealthy one. However, all of the larvae I checked last winter -- including those in mediocre (Ballingers Creek) and even poor streams (Moores Creek) were Prosimulium larvae. The question thus becomes -- is this more a matter of "time of the year" than the quality of the stream? Supporting that kind of conclusion is the fact that the larvae I checked from April through November this year -- in all of our streams, including the best -- were genus Simulium, one that is fairly tolerant. This is an issue I will follow closely in my stream visits this year.