Twenty-seven years have passed since Craig Mathews and I published Fishing Yellowstone Hatches.  At the time, our little book represented the most complete text available on Yellowstone insect hatches.  It remains so today.  But much has changed in the intervening years, and were we to publish the book today its list of insect emergences and their relative import would look quite different.  Following are some notes on the changes we’ve observed.  

The distribution of insect species and their population levels are never static.  Both fluctuate naturally, sometimes wildly so, for reasons not always well understood.  We don’t know for sure what’s causing the changes we see, but one obvious environmental difference between the 1970s (when we started observing area hatches) and now are warmer air temperatures.  This is true for both winter and summer. Warmer summer air temperatures raise water temperatures, and we suspect increased temperatures have played a major role in the changes we’ve seen.  The Madison and Henry’s Fork Rivers, two of the finest tailwater fisheries in the world and among the area’s most insect-rich rivers, have each experienced dam-related water flow issues for many years.  The effects of these aberrant flows on their respective insect communities are unquantified, but appear to us substantial.  There are likely many other significant short and long-term factors affecting insect populations throughout the region.    

Notes on specific emergences:

Mayflies:

Baetis tricaudatus:  Remains strong both spring and fall on the Henry’s Fork.   Fall populations on the Madison below Quake Lake have diminished greatly, yet remain important.  Firehole and Madison in the Park populations have declined somewhat, but still constitute important emergences.

Baetis punctiventris:  Historically, this was a major emergence on both the Firehole and Madison in the Park, stimulating huge rises of trout.  We haven’t seen a viable hatch on either river for many years, and would not include it in a book written today.  Substantial population declines on the Henry’s Fork too.

Pale Morning Dun:  Numbers have declined area-wide over the last couple decades, while remaining an important emergence.  The rivers that seem to have experienced the greatest declines are the Madison below Quake, Yellowstone, and Henry’s Fork.  

Green Drake:  For simplicity, we had grouped four species of fly under Green Drake—Drunella grandis, D. doddsi, D. coloradensis and Timpanoga hecuba.  The most significant change in the group has taken place on the Yellowstone.  Here, D. coloradensis (which is actually more closely related to a Flav than the true Green Drake, D. grandis), has gone from a roughly ten day emergence to one now lasting six or so weeks.  Above the upper falls, it’s by far the most important insect.  And it should be noted that the spinner falls of coloradensis are the important phase on the Yellowstone, not the actual emergences.

Brown Drake:  This hatch has disappeared from the Gibbon River.  Reports of their presence surface now and then, but when I’ve checked them out I’ve found only Gray Drake spinners (which can appear in minor numbers in wet years, and can be mistaken for Brown Drakes by the casual observer).

Flavs:  The Henry’s Fork remains the bastion of Flavs, though their numbers have decreased there.  We no longer see Flav hatches on the Firehole, Yellowstone, or the Madison below Quake, though a few specimens can be found on each river now and then.

Gray Drake:  Less prevalent on Slough Creek and the Yellowstone, but still important.  In wet years, Siphlonurus continues as a major hatch on the lower Henry’s Fork.

Callibaetis:  In our experience, populations of Callibaetis have always fluctuated in the Yellowstone area. No overarching trend in their numbers or distribution stands out.

Tricorythodes:  Similar to Callibaetis, populations have always varied from year to year, and continue to do so.

Pink Lady:  Epeorus albertae has transitioned from being a minor emergence (barely worth noting) on the Madison below Quake Lake, to being a very significant hatch.  E. albertae is a warmer water mayfly, and whether its rise in numbers is due to warmer weather or the years-long regime of surface flows from Hebgen Lake is unknown.  Emergences and spinner falls both bring trout to the surface, and Epeorus has replaced the Flav as the major evening mayfly on the Madison.  Epeorus were less numerous in 2018, when the river was once again fed from the bottom of Hebgen Lake.

Attenella margarita, Serratella tibialis:  Neither species of mayfly warrants an inclusion in a book of hatches today.

Heptagenia:  For the last 5-7 years there have been notable autumn populations of Heptagenia emerging from Soda Butte Creek and the Lamar River.  No mention would be given today to Heptagenia on the Yellowstone.

Caddisflies:

Nectopsyche:  Readers familiar with Fishing Yellowstone Hatches will recognize that this caddis doesn’t appear in our book.  That’s because back in the day, Nectopsyche (White Miller, colloquially) were nothing more than a token presence in the Firehole and Madison River (in the Park).  As the Firehole has warmed over the years, populations of Nectopsyche have exploded, essentially becoming a “new” hatch for fishermen.  Nectopsyche are far and away the dominant caddis on the Firehole, with massive numbers appearing yearly.  Emergence takes place from early June through September.  Numbers on the Madison fluctuate wildly year to year.  Both emergence and egglaying phases are important.  I’ve had two professional entomologists attempt to identify this fly to species level for me.  Neither was confident enough in their assessment to offer a judgment.        

Brachycentrus:  Mother’s Day caddis have long been notorious for year to year population fluctuations. They remain that way.

Hydropsyche:  The Hydropsyche complex (we covered four species in Hatches) have historically been the most important caddisfly genus for anglers in this area.  They appear to be suffering the most serious population declines of the caddis we cover.  These declines are not river specific, but rather area-wide. Still important to anglers and trout.

Helicopsyche:  No particular trends noticed.   

Glossosoma:  Populations on the Madison seem to be fluctuating more now than in years past.

Oecetis:  I don’t have enough recent experience to comment on the state of this hatch.    

Cheumatopsyche:  Like their Hydropsyche relatives, numbers seem down on the upper Henry’s Fork.  The Gallatin River in Yellowstone Park and Montana also has a fine July Cheumatopsyche emergence (we didn’t  write about it in Hatches), and for the last five or so years it has been trending stronger.

Lepidostoma:  Numbers much reduced on the Yellowstone.  

Arctopsyche:  Remains abundant on the Madison.   

Hesperophylax:  This beautiful caddis is now scarce enough that we would no longer include it in a book of hatches.

Rhyacophila:  Area-wide, numbers have declined dramatically.

Micrasema:  No longer prevalent enough to constitute a “hatch”.

Mystacides:  There have been significant habitat changes on the Henry’s Fork where this caddisfly was historically important, but they are still present in decent, if somewhat reduced, numbers.

Stoneflies:

Salmonfly:  Well known for dramatic population fluctuations.  No obvious trends.

Golden Stone:  No obvious trends.

Little Yellow Stoneflies:  Appear to have declined on the Madison, but not dramatically.

Damselflies:  Numbers on Henry’s Lake are much reduced from historic levels.  Elsewhere, we’ve noticed no real trends.

Midges:  Spring numbers on Hebgen Lake remain strong, summer numbers are down.  Midges remain significant year-round on rivers like the Madison and Henry’s Fork.