The other day I found several winter stonefly adults (order Plecoptera, family Taeniopterygidae) crawling around on a tree in a hardwood forest east of Blissfield, MI. They were about fifty feet from the River Raisin, the primary waterway in this area. These insects are normally found near streams and rivers because the naiads (immature forms) are aquatic. The young live in the water for several months to several years, feeding and growing. Near the end of their lives they crawl out of the water and molt into their adult forms (shown here). They then spend perhaps one to three weeks mating before they die.
I was somewhat surprised to find stoneflies in this location. This stretch of river suffers from significant contamination from agricultural runoff and enteric bacteria, and stonefly naiads are considered very sensitive to poor water quality and low oxygen. During the River Raisin Watershed Council’s biannual Adopt-a-Stream searches, stonefly naiads are never found near this part of the watershed.
The answer to my intriguing find comes from Bouchard (2004) who states “Most species of this stonefly family are generally intolerant to pollution, but some species are well adapted to large polluted rivers.”
Because most stoneflies are sensitive to pollution, their presence is typically seen as an indication of relatively clean water. Finds like this, however, show that there are exceptions to this general rule.
Bouchard, R.W., Jr. 2004. Guide to aquatic macroinvertebrates of the Upper Midwest. Water Resources Center, University of Minnesota, St. Paul, MN. 208 pp.