The Common Burrowers

By Clarke Garry

When talking insects with anglers and conservation groups, the Hex is a species about which there are almost always questions.  Many people have observed or heard about the impressive hatches of these adults that occur along lakes and large rivers.  And its ups and downs through recent time are a "wonderful mystery" (Humphrey pers. comm.) that we can now begin to appreciate.

When I initiated my benthic macroinvertebrate work in the mid-'90s, I eagerly anticipated collecting the larval stage of this insect, known as the common burrower.  Now-retired colleagues who had taken classes to the Upper Kinni in the early '70s recalled finding mature larvae nearing two inches in length.  What a find that would be!  When recent sampling produced only a few small larvae in expected locations, including targeted backwaters thick with silt, my questions began.  And now I'm understanding that an insect in decline can be an indication of improving water quality.

Plentiful and consistent Hex hatches on the Kinni are referred to as far back as the mid-1940's by Humphrey (1989).  Humphrey and Shogren (2001) write of the decline of the Hex and detail possible reasons: "The disappearance of the Hexagenia may be the result of improved water quality - falling water temperatures, a reduction in siltation, a speeding of the flow . . ., or other factors beyond our ken."  Interestingly, Edmunds and others (1976) build a case for the inflated hatches once seen, ". . . it is almost certain that the great masses of mayflies . . . are a symptom of man's unknowing influence with the environment.  Modern man has enriched streams and lakes with sewage from cities, manure and fertilizers from farms, and natural nutrients from eroding soils."  Today we see the Kinni responding to earlier stream protection projects, such as fenced easements, which restrict cattle access and encourage bankside vegetation.  This protection has lead to a reduction of the impacts expressed by Edmunds, including the important result of lowered water temperature and a shift in the insect community (Engel pers. comm.).

The best known of the burrowing mayflies is Hexagenia limbata (Ephemeroptera: Ephemeridae).  Its reputation is based both on large size (larva and adult up to 32 mm without tails) as well as large hatches.  Larvae inhabit silt-based substrates and prefer slower, backwater areas.  These larvae are extremely efficient diggers that produce U-shaped tunnels in soft bottom sediments.  As with many animals that live in aquatic burrows, the larvae propel water by rhythmically waving their gills (Edmunds et al. 1976).  From this flow they can filter detritus as well as extract oxygen.  Pre-adult subimagoes (duns) emerge from late June into August.

There are four species of the genus Hexagenia in Wisconsin.  All have an assigned tolerance value of 6 (based on a ten point scale, 0=excellent, 10=very poor) (Hilsenhoff 1987), i.e., a hypothetical biotic index sample composed entirely of Hexagenia would rate water quality as fair.  This is in striking contrast to today's common Kinni mayflies such as Ephemerella inermis, Baetis tricaudatus, and Stenonema vicarium which have tolerance values of 1, 2, and 2, respectively, and all of which indicate excellent water quality.

[I acknowledge and appreciate informative discussion and correspondence on this topic with Marty Engel, Roger Fairbanks, and Jim Humphrey.]

References:

Edmunds, G. F., Jr., S.T. Jensen, and L. Berner.  1976.  The Mayflies of North and Central America.  University of Minnesota Press, Minneapolis, 330 pp.

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

Humphrey, J. R.  1989.  The Kinnie.  Fly Fisherman Magazine, July, pp. 32-35, 59-61.

Humphrey, J. R. and W. D. Shogren.  2001.  Trout Streams of Wisconsin and Minnesota, 2nd ed.  Backcountry Guides, Woodstock, VT, 302 pp.

Addendum:

One of the purposes of my benthic sampling and databasing project is to develop a permanent record of the macroinvertebrates that are living now in the Kinnickinnic River.  I am also interested in documenting entomological changes where possible.  I am asking interested anglers to send me any accounts of past hatches, particularly where dates and locations are included, that I can archive as part of my macroinvertebrate records.  These will hopefully include, but are not limited to, the Hex hatches.

I would like to receive written Kinnickinnic River and tributary accounts by email (clarke.garry@uwrf.edu) or letter (Department of Biology, UWRF, 410 South 3rd Street, River Falls, WI 54022) and would appreciate an accompanying statement granting me permission to use your account online or in written reports (in which I would acknowledge the contributor).  Details regarding this request can be found at:  www.uwrf.edu/~cg04/Kinni_page2000/hatch_accts.html

Dr. Clarke Garry of River Falls, WI is a professor of biology at the University of Wisconsin- River Falls