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Aquatic Insect Declines?

Updated: Feb 17, 2022

The question mark hopefully tips the reader off that there is some speculation but I will try to bring in some peer reviewed science, some data from Wisconsin streams, and personal experiences of myself and those I know that fish a lot in the Driftless Area. I am less familiar with what other parts of the state might be experiencing but I would interested in your comments about your experiences in the Driftless and elsewhere. (comment at the bottom of this post)

Hexagenia limbata dun
Hexagenia limbata dun on a fishing trip to Central Wisconsin

To be sure, it is not wild speculation that insects are declining in many places - it has been documented in prestigious journals like Science, the Entomological Society of America recently produced a symposium on the topic, the phenomena has its own Wikipedia page, and has been widely written about both in scientific journals and in sources intended for a wider audience (see links at the end of this post). Anecdotal evidence - such as the "windshield phenomenon", fly anglers noticing reduced hatches, blog posts, and other sources of anecdotal evidence abound. Certainly the decline in pollinators and Monarch Butterflies has been front page news.

As with most global questions, there is not a single smoking bullet nor do all locations demonstrate the same the same trajectory. Nature is never that simple. The causes are many and varied depending upon the location and local stressors. As one ecologist put it, it is a death by a thousand cuts. The causes are many of the standard causes for rarity and decline (HIPPO) - habitat destruction, invasive species, population, pollution, and overharvesting. Climate change, really a function of human populations, is a commonly cited explanation for part of these changes. Pollution is what has probably received the most most attention. In particular, insecticides and herbicides have received the most attention. Neonicotinoids have certainly received a lot of attention due to their effects on bees and other pollinators and are proposed to be one of the causes of insect declines. For aquatic insects, certainly these agricultural pollutants make their way into the water but sedimentation and organic pollution may be just as important sources of aquatic insect declines.

While there was been an overall increase in aquatic insects (pdf of Science paper), that increase has not been universal and the upper Midwestern United States is one of the areas experiencing decline. The paper linked above is a global meta-analysis meaning that the authors took the data from a number of published papers and re-analyzed the combined data. It is a powerful way to look at "big picture" trends - such as those occurring over time and at large spatial scales. You will notice from the map above that the analysis on aquatic insects - which showed an increase - is not universal. It also demonstrates one of the great issues in "global" analyses - they are biased by where research has been conducted and published. It is quite evident that this global analysis is largely confined to the Northern Hemisphere and particularly to Europe and North America.

Not to go too far into the science but I think it is important a bit about how scientists quantify declines. For one, typically these discussions revolve around total abundance - simply an estimate of the total number of individuals. However, this only tells part of the story as declines in one species may be counteracted by increases in another. If the declines are seen in species that are intolerant to pollution and the increases are seen in those that are more tolerant to environmental degradation, total abundance does not really tell much of the story. Scientists have ways to tell those stories through measuring other characteristics of communities. If you are a trout, you may not care so much that total abundance is down if the macroinvertebate (note macroinvertebrate refers to insects as well as crustaceans like scuds and other non-vertebrate, non-insect taxa) biomass remains the same or even increases. Additionally, scientists often look at biodiversity which is a measure of the number of species as well as each species abundance.

Proportion of species within each order that are extinct, endangered, vulnerable, or in significant decline. Order names in the figure are: Ephemeroptera = mayflies, Odonata = damsel and dragonflies, Plecoptera = stoneflies, and Trichoptera = caddis. Quite obviously, most of the aquatic insects we are most familiar with are unlikely to be those that have gone extinct or are endangered or vulnerable. Source:

Closer to "home", the Hexagenia mayfly hatches on Lake Erie and the Mississippi River are the thing of legend. Hell, the National Weather Service has a page devoted to the mayfly hatch on the Mississippi River and the folks on Lake Erie take their return very seriously! These hatches have been on the decline (pdf link to PNAS article). Lake Erie was at one time a huge success story because after decades of essentially no mayfly hatches, they returned to the lake as water quality improved thanks to the Clean Water Act. Unfortunately, in 2020, pollution and algal blooms have reduced the hatches to fractions of what they had been (National Geographic).

The study revealed that between 2015 to 2019, populations of burrowing mayflies in the genus Hexagenia declined by an incredible 84 percent in western Lake Erie. In the nearby northern Mississippi River Basin, from 2012 to 2019, they declined by 52 percent.

The declines are quite staggering and represent a huge loss of biomass and food for the fishes, birds, spiders, bats, and other animals in and around Lake Erie and the Upper Mississippi River. Canaries in the coal mine? We shall see but the declines in mayfly populations, declines of several billion individuals - yes billion! - is certainly concerning.

Enough Science, Let's Get Anecdotal

I think I have demonstrated the there is some pretty good evidence of the decline of insects and that changes in macroinvertebrate communities are spatially variable and caused by a variety of anthropogenic (human caused) environmental stressors. It is a lot more difficult to find local data on insect decline unless someone has taken the considerable time and effort to collect and analyze the data and then publish it. A quick Google Scholar search provides little scientific evidence for aquatic insect declines in Wisconsin - at least in the scientific literature. Of course not everything that happens in nature makes its way into the scientific literature. What we do have is a lot of anglers, many of whom have a pretty good amount of experience and knowledge of insect hatches and how they have changed over time.

Coon Creek, Driftless Area, Wisconsin
An ideal setting - wide prairie buffers that supply terrestrial insects, prevent erosion, and deter a lot of anglers.

My experiences will be mine, different from yours, but together the experiences of 10's or 100's may well add up to something pretty useful and significant. In my experiences, the early midges (Chironomidae) and winter stoneflies (Capniidae and Taeniopterygidae) are relatively unchanged - they are there, and at times important, but generally not what a dry fly snob would call dependable. The Grannom / Black Caddis hatch (Brachycentridae family - quick note - for animals, the "-idae" suffix means it is a family) is still dependable, in fact I generally have a 50 plus fish day or two in the Driftless most years on this hatch. This is generally the hatch of the year, at least for me. The Hendrickson hatch (Ephemerella subvaria) is not what it used to be in the Driftless - though others in Central and Northern Wisconsin are witnessing better Hendrickson hatches. The tan caddis (family Hydropsychidae) in a size #14 to 16 is not a "blizzard" hatch but are very dependable from sometime in early to mid-May until mid-June or so, depending upon the year. It is my favorite hatch to fish. The March Browns (Maccaffertium vicarium) were always fairly hit or miss but their robust spinners were a fun late evening hatch I have not fished effectively in a few years. The last of the spring in to summer transitional hatches was always the sulphurs (holy cow, are there a lot of ways I have seen this spelled). Depending upon where you are and the time of the season, the "sulphur" hatch may be one of a few different Ephemerella species (E. invaria and E. dorothea - and since I have not kept up on the taxonomy, maybe others). The sulphur hatch in my experiences went from being "the hatch" to a hatch, maybe, on many Driftless streams. Friends on the Wolf River and other places say that their sulphur hatches are on the decline as well.

A Driftless stream riffle
A "bug" producing riffle - some of my favorite places to fish are these riffles and the pools or runs they feed.

In my experiences in the Driftless, the most dependable hatches are the several species of "blue-wing olives" which are a pretty diverse group of species, generally - but not always - of the family Baetidae. Their biology is different from many mayflies because some species are multi-generational (more than one generation per year). The seasoned angler knows to expect BWO hatches when the weather is overcast to rainy. The BWO hatches can be experienced many months of the year, really from March through the end of the season which is now mid-October. Know that throughout the year, what we call BWO's will range from, oh, a size 14 to a 24 or smaller. Sizes generally are small in March and April, increasing in May and June, generally less common in July and August, and the smaller species / generations pick up in late-August through the end of the season. And while they are generally dependable, look for them on the days with fog, rain, and even snow. It is not always the most pleasant hatch to fish...but it is dependable, if you are there at the right time. Yes that sounds a little like every hatch but BWO are more weather dependent than most.

Mid-summer and late-summer hatches have always a little hit or miss. The Driftless tends not to get many later season mayfly hatches (Hexagenia, Ephemera, Potomanthus (now outdated), Ephoron, and other mayflies that are typically burrowing mayflies. It is thought that the Driftless with its floods and shifting sediments is not well suited for long-lived burrowing mayflies - though locally, many streams have/had dependable hatches of these mayflies. In the summer - what I would consider mid-June (once it starts hitting 80*F air temperatures) regularly through August - hatches have always been less dependable. The Yellow Sally stonefly (Isoperalla spp.), light cahills (a very diverse group), and other mid-summer hatches have, in my experiences, been hit or miss, but really fun when you hit them.

Pile of CDC and Elk flies
Hans Weilenmann's CDC and Elk - the only fly you might need for about 50% of the season.

This post has certainly been mayfly-centric - as much of fly fishing tends to be - for better or worse. Certainly some of the best Driftless (and Wisconsin) hatches have always been the caddisflies and in general, they have been overlooked. In my experiences - which will make me sound older than I feel - the caddis hatches are as good as they have ever been. The cranefly hatches have always "been there" but now are a feature of the Driftless (for many, they always have been). We used to think we were pretty clever knowing and fishing the May through mid-June craneflies that few seemed to know about. The craneflies, and there seem to be a few species - the yellow ones hatching before the orange ones - are much more poorly documented. People that know a hell of a lot more entomology than me given them really poor treatment in books and articles. Ann Miller, author of the best book for hatches for Upper Midwest streams simply refers to Tipulidae and really only talks about the big craneflies. The yellow and orange cranes I encounter are size #14 on the large side and #18 on the small side with #16 being the size I fish most often. I need to do a bit more research on what our craneflies really are as they do not seem to be well documented.

Like other places, I would suspect that the cause of the declines of hatches I've experienced is another death by a thousand cuts. Reduction is Conservation Reserve Program (CRP) lands and more acres of row crop agriculture means more agrichemicals making their way into Driftless streams. Climate change, particularly the frequent recent floods, have certainly been density-independent events that "knocked back" insect populations. With several of the streams I fish most often having experienced "100 year floods" in consecutive years and a number of floods in the last 20 years, the "bugs" might not have had sufficient time to recover. Or maybe it is mostly bad timing on my part. There is always a bit of luck or good fortune involved in witnessing a really good hatch. Whatever the cause(s), I miss the days where the sulphurs were dependable and two hours of fishing could be enough to make the day a huge success.

Comment below about your experiences, wherever you may live. I would be curious what your experiences with hatches have been. Thanks!

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I have fished the Hexagenia hatch on the rivers of NW Wisconsin for over 55 years. Although they are still an impressive natural occurrence, there is no where near the volume of bugs that I witnessed during the 1960s and 1970s.

Andrew Voorhees
Andrew Voorhees
Dec 25, 2022
Replying to

Reductions in hex populations can actually be a sign of stream improvement considering the type of habitat required for them. Again, as this article has discussed, understanding why a certain species may be in decline or increasing in population can be a complex issue including water flow, water temps, stream erosion, pollution (type and concentration of pollution), food sources and more. I know on my home waters, the hex hatch has decrease over the past 2 decades but in this case it was probably due to colder water flows as a result of city/DNR/TU and other groups that continuously have tried to improve steams in the area. Keeping a diary with water temps, flows, hatch activity (species and concentration based…

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