top of page



An Observant Approach to Fly Selection

Updated: May 22

Probably the single most frequent question asked in any fly shop or on any guided trip is some version of “What’s working?” To answer this question requires knowledge of what’s available. What’s on the menu? If it ain’t on the menu, you might get a curious trout to eat it, but it’s not likely to work as consistently as a fly imitating the most abundant natural at any given time. For most consistent success, you need to first know what’s available. So much of fly fishing is derived from this origin point. Furthermore, the most curious minds might ask, “What’s most available?” That’s a better question. More availability, it’s safe to assume, likely equates to more likely to eat. So it’s first important to understand your river’s benthic macroinvertebrate community (of which we know quite a bit about on the Upper Madison - read that post HERE), and what bugs have the highest “relative abundance.” Relative abundance is a measure of how common or rare a species is relative to other species in a defined location or community. In this case, the Upper Madison river. Relative abundance is the percent composition of an organism of a particular kind relative to the total number of organisms in the stream. 

And when we understand what bugs have the highest relative abundance, we have a basis on which to select our flies depending on the season. I say depending on the season because, even though we know what’s most abundant in a stream, the season often dictates what’s most available. This is because all those aquatic bugs go through life cycles, and at various times in those life cycles they become far more available than other times. 

So let’s make it less complicated. Assuming an angler understands what’s in the stream in abundance, depending on the season they can make an educated guess as to the best fly pattern to choose for success. Let’s break the Upper Madison into 2 areas: immediately below Hebgen and in Ennis. The top and bottom floors of the river. Next let’s examine the top 5 most abundant species of bugs (and the simpler Order they belong to - mayfly, stonefly, caddisfly) in those sections. 


The top 5 most abundant species (% relative abundance) of aquatic insects on the menu for trout in Ennis (according to 2023 surveys) are:

  1. Helicopsyche borealis (17.2%) - Caddis

  2. Baetis tricaudatus (10%) - Mayfly

  3. Glossosoma sp. (9.8%) - Caddis

  4. Tricorythodes sp. (8.1%) - Mayfly

  5. Zaitzevia sp. (5.5%) - Riffle Beetle


The top 5 most abundant species (% relative abundance) of aquatic insects on the menu for trout  just below Hebgen Dam (according to 2023 surveys) are:

  1. Attenella margarita (23.3%) - Mayfly

  2. Cricotopus sp. (16.8%) - Midge

  3. Optioservus sp. (10.3%) - Riffle Beetle

  4. Drunella grandis (5.4%) - Mayfly

  5. Phaenopsectra sp. (3.4%) - Midge

A couple things might surprise many people. First, the Upper Madison is often closely associated with stoneflies, namely giant salmonflies. But also golden stones, yellow sallies, and skwala. So where are those guys here? They’re around for sure, but in most cases stoneflies represent less than 3% of the relative abundance of all bugs at any given site on the Upper Madison. That may be surprising, but it’s no different than elephants being giant animals but making up low relative abundances on the African landscape. Doesn’t mean they’re not important, just less abundant. Smaller critters are typically more abundant. 

Secondly, riffle beetles eh? What are those? Turns out riffle beetles are very abundant in portions of the Upper Madison, and get overlooked as an important food source and fly choice. 

Riffle beetles, like a caddis, undergo complete metamorphosis, which means they go from eggs to larvae to pupa to adult. Some species of riffle beetles complete their life cycle in the course of a single season, but some can live for three or even five years. Here’s the most important point: riffle beetles may be active at any time of the year. It’s not hard to find the larvae, which look like oversized midge larva, and they squirm like a mayfly nymph. If you’ve turned over a rock in Ennis before, you’ve likely seen them, but probably mistook them for a mayfly nymph. Here’s a picture of one at Valley Garden I took last week. It's hard to tell in the picture, but this is about a size 14 bug.

Being fully aquatic, riffle beetles (adults and larva) can stay underwater for extended periods or even indefinitely. Want to throw that fly shop bro for a helluva loop next time? Ask him if he's got any beetle nymphs. The larva obtain oxygen via the hairlike gills at the abdomen tip, while the adults breathe through a process called plastron respiration, in which they garner oxygen from the water using the hairs on their body. Both larva and adults require high levels of dissolved oxygen in the water. In other words, healthy streams. These conditions are often found in cool, shallow, swift streams. Sounds like the Upper Madison to me. 

Third, major lack in caddis in the assemblage at Hebgen (all caddis only 4% of all bugs!). Interesting. So, the bug assemblage near Hebgen is very different and your fly selection should be too. Midges are really important up there. It’s also a reach on the Upper Madison where western green drakes are most abundant. A single species of tiny BWOs (Attenella) is a hugely dominant mayfly there. And look at that, riffle beetles are even more important near Hebgen. 

But let’s simplify it a bit more. In Ennis, roughly 1 in 4 bugs available is a caddis. That’s a lot. Roughly 1 in 5 are mayflies. So from a nymphing standpoint, you can’t really go wrong with a caddis and mayfly nymph imitation. And thankfully, most mayflies and most caddisflies have a pretty similar size and appearance. Similarly, at Hebgen, nearly 1 in 3 are mayflies and 1 in 5 are midges. Smaller nymphs near Hebgen likely work better. 

Now that we understand what bugs are most abundant, does that mean you should always choose a caddis (adult or nymph) as a fly in Ennis? Not necessarily. For example, many caddis larva, although massively abundant in Ennis, are not frequently available to trout. Many are case makers of one sort or another. Take for example the two most common caddis species in Ennis, Helicopsyche and Glossosoma. Both are casemakers. The former a snail shaped case maker and the latter a saddle case maker. Larvae of Helicopsyche live mostly inside these cases and rarely are exposed, feeding while still in the case. Thus, there aren't really any larval imitations of this caddis. Glossosoma, although similarly living as a larva in a case of rock and sand, come out of their cases as they grow bigger. When they outgrow one case, they leave and build another. This episode has been documented to be somewhat synchronous. Meaning, many of them do this at roughly the same time. As they leave to build new homes, they can become drastically more available to trout as some get swept into the drift. The larva are creamy pinkish. And patterns imitating this can be deadly when this event is happening. But for both species, the most available they will ever get to trout is after they’ve matured into pupal form and make their way to the surface prior to hatching into adults. 

And that brings up the second most important variable in choosing a fly or flies. Seasons. Different bugs have different seasons in which they are more active and thus more available. 

This, for both species of common caddis in Ennis on the Upper Madison, typically happens from late June throughout July and somewhat in August. So, the absolute best time to fish a caddis pupa in Ennis would be in the summer. Probably not a top pattern in winter for example.

Mayflies, however, don’t have a case and are active on the bottom and available to trout year round. In Ennis, for example, Baetis tricaudatus is far and away the most common mayfly species. Imitating this nymph is effective year round. Darker thorax versions are even more effective during hatch times, typically from May-July. The thorax darkens on nymphs as they mature and rise to the surface to hatch. 

Take now for example. In the heart of Spring. If I’m fishing in Ennis today I might consider which of those most abundant bugs are going to be most active. Meaning, are any of those bugs hatching and thus more active and available in the water column? Currently, Baetis tricaudatus (BWOs), another mayfly species Rhithrogena (March Browns), midges, and a lesser abundant caddisfly but seasonally important Brachycentrus (Mother’s Day caddis) are all actively hatching and available to trout both as emerging pupa/nymphs and as adults. Trout are seeing these bugs right now more than any other regardless of relative abundance, and thus a likely primary food source right now. Focus on these flies on the Upper Madison in Ennis is likely a smart play. 

There are some exceptions to this scientific strategy. Let’s go back to stoneflies, everyone’s favorite. Though they’re not actively hatching right now, they’re still active on the bottom and available to trout. More importantly, they’re very conspicuous. Very large. Prime ribs. And although stonefly relative abundance is around 4% in Ennis, that doesn’t tell the whole story. Stoneflies, unlike mayflies or caddis, live in Upper Madison river as eggs for 1 year and nymphs as 3 years. According to 2023 data, there are 6 species of stoneflies in Ennis. That’s 18 different age classes (6 species times 3 age classes each) of stoneflies in the river right now ranging from probably a size 12 or 14 to a size 2. So, stoneflies are easily seen by trout due to their size, offer orders of magnitude more protein than a small mayfly, are not very quick moving and thus easier to prey on, and are available in a wide range of sizes. This is why 365 days out of the year, a stonefly nymph is always a top option on the Upper Madison, regardless of season or hatching. And paired with a seasonally appropriate caddis pupa or mayfly nymph, this combo likely takes more trout than any other rig on the river. 

And if you’ve ever fished nymphs with me before, you know how much I value the stonefly nymph. So much so that I double up often. Two big stonefly nymphs. I do this for a couple of reasons. First, as I just mentioned, what self respecting trout wouldn’t pass up the opportunity to eat a protein rich, slow moving stonefly. They absolutely love stoneflies. Secondly, I hate adding weight to my nymph rigs. I haven’t used a split shot on a nymph rig in a decade on the Upper Madison. That’s the honest truth. I love simplicity. So rather than use split shot in higher water times, I often just use another size 4 girdle bug which is weighted with tin under the materials. What’s better than one prime rib? Two prime ribs. The combo offers quick trips to the right depth, tighter connections to the leader, less slack, more visible offerings in high water for trout, and very obvious, non-subtle strikes. 

But for the angler who wants to try and “match the hatch,” a cliche for sure, but an accurate one, an easy way to figure out what’s available to trout, and floating freely in the drift is to carry a small drift net. Simply hold it in the water for a few minutes and collect what’s in the water column. Study that and select flies in your box that match what’s available. But understanding what’s present and abundant based on season will give you the knowledge to walk into a shop and purchase the bugs that will be likely relevant when you get in the water. Nothing sucks more than sampling a river, looking at your box, and discovering you don’t have anything that closely resembles the available menu. So study, be prepared, and be observant for more success. It’s a really rewarding experience to put it all together. And don't forget your riffle beetles!


bottom of page