The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
I'm not sure if you have the answer for this question. However, I'll ask anyway, maybe you have some idea of what's happenning!
In S.E. Minnesota from the mid 70's til today, we have lost a number of great hatches on MANY streams.
We have lost Ephemerella dorthea, Pseudocloeons, Stenonema Vicarium and Ephemerella Subvaria and Invaria & Rotunda's hatches that were once very impressive. These hatches are now very light or sporadic in most years and the Dorthea hatch is totally GONE!! Roger, what do you feel is causing the loss of these hatches? On most of the streams where these hatches were once prevalent, the in-stream substrates/habitat doesn't look any different then in years past! In other words the amount of silt in the stream bed is about the same as it always has been! The only thing that I feel has changed over the last thirty years is most of our watersheds have been tiled out and there has been an increase in row cropping of corn & soybeans!
Any ideas why we are losing our "BUGS"
Let me preface this response by saying I have no specific information about the trend of mayfly hatch density in SE Minnesota which would confirm your observations, nor for that matter, any information which would rebut your observations.
However, assuming your observations (as stated above) are essentially accurate, my guess as to the primary cause would be a water chemistry change detrimental to mayfly reproduction. And, one obvious suspect would (of course) be increased agricultural use of insecticides. I would recommend discussion with an entomologist in your state's department of ecology in order to explore the likelihood of this being the cause.
Many of the most treasured mayfly hatches (like the Hendricksons) are canary-in-the-coal-mine species
Jason, I know that when benthic invertebrates are surveyed as a tool to assess the overall health of a stream or section, they are given a rating based on their relative sensitivity. Do you know if these ratings are sufficiently species-specific to provide a point of comparison with my (admittedly anecdotal) observations?