The Little Yellows
By Clarke Garry

There are normally several logistical considerations associated with making subsurface insect collections in the Kinnickinnic River in January, not the least significant of which is removing the creatures from the sorting tray before the motionless water progresses through the slush stage on its way to becoming ice. Once the assemblage is acquired, however, the collector will be rewarded with an interesting diversity of macroinvertebrates, providing evidence of winter insect activity and the promise of coming spring and summer hatches.

Midwinter Kinni collections collectively yield four categories of stonefly larvae (nymphs), each distinguishable by size and color. These are: winter (=little brown) stoneflies (8-12 mm excluding antennae and tails, dark brown to black), giant stoneflies (35-50 mm, dark grayish brown), common stoneflies (15-25 mm, brown with tan markings), and little yellow stoneflies (10-14 mm, yellow to tan, with darker tan to brown markings on the head and thorax, and a striped or spotted abdomen).

The little yellow stoneflies that inhabit the Kinnickinnic River are members of the genus Isoperla (order Plecoptera: family Perlodidae). Hilsenhoff (1995) lists 12 species of Isoperla from rivers across the state of Wisconsin; four of these have been found to date in the Kinni. These are: I. slossonae, I. transmarina, I. bilineata, and I. dicala. Of these I. slossonae and I. transmarina larvae are collected most often and in highest numbers throughout the lower Kinni region. I. bilineata is known as a larger river inhabitant and is collected only in small numbers in the downstream regions of the lower Kinni. I. dicala is rare.

Stoneflies in general are a sign of high quality water and this is no less true for the little yellows. Biotic indexing tolerance values assigned to these four local residents are: (based on a ten point scale, 0=excellent, 10=very poor) 0 for I. transmarina, 2 for I. slossonae and I. dicala, and 4 for I. bilineata (Hilsenhoff 1987). With the exception of an occasional isolated specimen upstream, the uppermost occurrences of little yellow nymphs appear to be the Quarry Road-Liberty Road areas. This distribution pattern is similar to one that I've observed and reported for several mayflies, caddisflies, and other stoneflies.

At the time these larvae are observed in January, they're only a few months away from hatching. This adult emergence will occur between mid-April and late June. Hilsenhoff and Billmyer (1973) report a sequence of hatches for Wisconsin as follows: I. slossonae emerging mid-April through May, I. bilineata and transmarina mid-May to mid-June, and I. dicala late May to the end of June. These species have a one-year life cycle, spending the summer months as eggs, which hatch in late summer or early, fall (Hilsenhoff and Billmyer 1973). Collections from the Kinni are consistent with this generalization, as I have not found any of these species as larvae in the river between 9 June and 9 September.

Although most species of Isoperla are carnivorous as larvae (Hilsenhoff 1995), earlier studies referred to by Stewart and Stark (1993) suggest that I. bilineata is an herbivore. The little yellows themselves become objects of prey and available to fish as they make their way to the shoreline prior to emergence. Hafele and Roederer (1995) make this comment on adult little yellows: "Look for the females on warm summer evenings when they frequently form large swarms over riffles and runs to lay their eggs. As they gently glide to the water's surface, trout wait below, eager to intercept them."

References:

Hafele, R. and S. Roederer. 1995. An Angler's Guide to Aquatic Insects and Their Imitations, Johnson Books, Boulder, Colorado, 182 pp.
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.
Hilsenhoff, W. L. and S. J. Billmyer. 1973. Perlodidae (Plecoptera) of Wisconsin. Great Lakes Entomologist 6:1-14.
Stewart, K. W. and B. P. Stark. 1993. Nymphs of North American Stonefly Genera (Plecoptera), University of North Texas Press, Denton, Texas, 460 pp.

Dr. Clarke Garry, of River Falls, WI, is a professor of biology at the University of Wisconsin - River Falls. This article is one of a continuing series on the aquatic fauna of the Kinnickinnic by Dr. Garry.