Lets Talk Bugs
By Tom Lager

Quite a number of years ago, while exploring along the stream that flowed through our farm in northern Minnesota, I saw a migration of critters that I have never witnessed since. It was spring and the high flows from the snow melt had subsided. I saw a hundreds of little wiggling critters swimming upstream. They followed the banks on both sides of the stream in a caravan fashion — not in an orderly procession, but in a band of individuals several inches wide. I did not know what they were, so I scooped some into a container to watch them closely. They were so numerous it was easy to capture a plentiful group in one scoop.

At the time, I did not know they were mayfly larvae. I knew they were immature insects of some kind, but did not know their adult form. Through later experiences, I surmised they were mayfly larvae, most likely Leptophlebia larva. They swam around in the jar, wiggling in their typical undulating fashion. Their behavior and large numbers made a big impression on me, such that a number of years later while in graduate school I recalled the event and was able to understand what I had witnessed.
As shown in the figure they are well adapted for swimming. Notice the well developed gills and long tail filaments. Several reports in the literature document significant upstream migrations from lakes into tributary streams. I expect this behavior is rare or at least infrequently observed, since few reportings have occurred. Can you imagine the trout reaction if this were to occur in a trout stream.

Let me know if you have made similar observations.

Invertebrate behavior is very interesting. The current issue of Midwest Fly Fishing has an article by Ann Miller, entitled Invertebrate Drift: Under Cover of Darkness, that is worthwhile to read. She reviews the phenomena of downstream behavioral drifting of bugs immediately following dusk and preceding dawn. This behavior persists for several hours as they release themselves into the current and disperse downstream seeking new habitats. She discusses the paradoxical question of why upstream habitats do not become depleted of bugs due to behavioral and accidental drifting of invertebrates and cites a well written book by Thomas F Waters. Walters is a retired Fisheries Professor from University of Minnesota (35 years of dedicated research on flowing waters and long-time Trout Unlimited member) who conducted pioneering research on drifting behavior in trout streams near St. Paul, Minnesota. His book is, Wildstream A Natural History of the Free Flowing River (Riparian Press, St. Minnesota 2000).

A comment regarding my experience with water temperatures in the upper Pine River in March: I had reported measurements of 45-46 F and thought this was lower than expected – at least for my limited exposure with this stream. I discussed this with Al Niebur (DNR) who has conducted significant research on this reach of the Pine and he assured me that the temperatures I recorded are very typical of the spring-waters flowing throughout the area. Al is very proud of all the great work that TU and the DNR have done on that section of stream.

As you enjoy your fishing this month, take the time to turn over the rocks and look at the bugs.