by Clarke Garry

I found myself standing in the lower Kinnickinnic River on 1 January 2001 as a partially ceremonial kickoff to another year of macroinvertebrate sampling. After a few kick net samples, I was reminded of a familiar mid-winter benthic collection, a winter stonefly, and the rather remarkable story of its emergence.

For entomologists and flyfishers alike the term "winter stonefly" implies any of a number of plecopterans of the four families Capniidae (slender winter stoneflies), Leuctridae (rolledwinged stoneflies), Nemouridae (nemourid broadbacks), and Taeniopterygidae (taeniopterygid broadbacks). So far, macroinvertebrate and biotic indexing collections have revealed small numbers of two species of nemourids in the Kinnickinnic system, and all of these in expected spring-proximal habitats (mainstem upstream from 140th Street and Kelly Creek). Far more common are members of the family Taeniopterygidae, particularly Taeniopteryx nivalis, the early brown or early black stonefly. To date no other winter stoneflies, notably capniids or leuctrids, have been found in the Kinnickinnic watershed.

These stoneflies are univoltine, i.e., having a single generation per year. Adults emerge in late winter and early spring; they mate and the females deposit their eggs in the water. Hilsenhoff (1995) fills in the details of the life cycle: "Larvae hatch from eggs almost immediately, feed briefly, and then burrow into the substrate where they spend the late spring and summer in diapause [a state of suspended growth and development typical of many insects, CG]. Mummy-like diapausing larvae resume a normal appearance in September and commence feeding . . ." Ongoing inventory work suggests that the earliest the larvae appear in the Kinnickinnic is late November, and regular collecting from that point on indicates growth toward maturity through the winter months. (The latest I have collected the larvae from the Kinni is the 20th of March.) Then a real treat is in store on just the right days [those with a combined optimal temperature and day length (Hynes 1976)] in February and March, when the small, dark adults emerge onto the surface of the snow! I know of no observer, including most aquatic entomologists, whose excitement is hidden when talking about these little winter emergers.

Taeniopteryx sp. larvae can be confirmed by locating a single finger-like gill at the base of each leg. T. nivalis is distinguished from other taeniopterygid species in its larval form (Fullington and Stewart 1980) by the presence of prominent light yellow margins on the sides of the pronotum and light rings around the eyes. Both of these color features show up well against the dark brown to black body coloration. The antennae, which are as long as the body, are dark brown at the base and transition to a tan-yellow; the two cerci ("tails") show a similar color pattern. The species is described in the literature as both possessing and lacking a light colored, mid-dorsal longitudinal stripe; those of the Kinni mostly lack this stripe. The larvae feed on detritus and diatoms. They are most commonly found in debris and submerged vegetation outside of the strongest current of the river. Stoneflies in general have low tolerance for organic pollution; Taeniopteryx species have a tolerance value of 2 (0=excellent, 10=very poor) (Hilsenhoff 1987) and therefore indicate high quality water. I’ve found T. nivalis in almost all of my standard collection locations; they occur from near the delta to north of I-94.

In years with an early spring, emergences of the early brown/black stonefly often end prior to the opening of trout season. In cooler years the hatches are delayed and can occur in-season (Borger 1980). We occasionally wonder on what trout prey in late winter and early spring. This winter stonefly is available in significant numbers during hatches of this period.

References:

Borger, G. A. 1980. Naturals, A Guide to Food Organisms of the Trout. Stackpole Books, Harrisburg, PA, 223 pp.

Fullington, K. E. and K. W. Stewart. 1980. Nymphs of the stonefly genus Taeniopteryx (Plecoptera: Taeniopterygidae) of North America. Journal of the Kansas Entomological Society 53:237-259.

Hynes, H. B. N. 1976. Biology of Plecoptera. Annual Review of Entomology 21:135-153.

Hilsenhoff, W. L. 1987. An improved biotic index of organic stream pollution. Great Lakes Entomologist 20:31-39.

Hilsenhoff, W. L. 1995. Aquatic insects of Wisconsin, keys to Wisconsin genera and notes on biology, habitat, distribution and species. University of Wisconsin-Madison Natural History Museums Council Publication No. 3, G3648, 79 pp.

Dr. Clarke Garry is a professor of biology at UW-River Falls.