The famous "Sulphur" hatch in July and August is an event that has for years frustrated and confused anglers on the three Paradise Valley spring creeks-Armstrong's, DePuy's, and Nelson's.

Typical summer fishing on the creeks is relatively easy in the morning, as the remnants of the Pale Morning Dun (PMD) hatch, Ephemerella inermis, bring trout to the surface and anglers take fish on nymph and dun imitations. But by midafternoon, when the "Sulphurs" take over the stream, the easy fishing quickly dissipates. Thousands of trout rise as the small, yellow-colored mayflies take to the air, but few anglers have success fishing dun imitations.

Anglers travel from all over to fish the summer's prolific "Sulphur" hatch on Montana's Paradise Valley spring creeks like Armstrong's

We have studied this hatch intensively from a fishing and an entomological viewpoint, and what we found can provide you with a clear understanding of the hatch so that you can improve your fishing success on the spring creeks and possibly other Western streams.

Proper Identification
First, you should know that the insect Paradise Valley spring-creek anglers call a Sulphur has nothing to do with the Eastern Sulphur, Ephemerella dorothea. In fact, it's not an Ephemerella at all, or the genus Centroptilum, as some local fly shops indicate on their hatch boards. Various writers have reinforced this misidentification over the years.

Centroptilum is found nearby in the mainstem Yellowstone River, which is generally much warmer in August than the spring creeks. While some of the Centroptilum spinners stray over into the neighboring spring creeks and can be positively identified, the insects have not established colonies on any of the three creeks. In extensive general benthic samples taken by co-author Gustafson, a Ph.D. entomologist working out of the Montana State University, not a single Centroptilum dun or nymph was found in the creeks.

The insect that hatches every August afternoon on the creeks is an ordinary Baetis tricaudatus or Blue-winged Olive (BWO). This fly is found all over North America, from coast to coast, as far north as the Alaskan tundra and as far south as northern Mexico.

W. Patrick McCafferty, the Purdue entomologist and author of Aquatic Entomology, believes that Baetis tricaudatus is "possibly the most ubiquitous mayfly in North America." Based on samples we sent to him, McCafferty confirmed that the Baetid found in the spring creeks in August is Baetis tricaudatus, not Centroptilum.

For many spring-creek fishermen, it will stretch the bounds of credibility to learn that the BWO and the "Sulphur" are the same species. But this phenomenon is not unique to the spring creeks. McCafferty says that some "Sulphurs" found in Wisconsin and Michigan are Baetis tricaudatus, and we suspect that the "Sulphur" version of this fly also occurs elsewhere in the Rockies.

How can this be? Baetis tricaudatus exhibits multi-voltinism, meaning there are two or more broods of the insect each year. On the Montana spring creeks, there are distinct hatches in early spring, midsummer, and fall. But the midsummer fly exhibits a different coloration than the spring and fall hatches due to differences in water and ambient temperatures. In August, the Baetis no longer looks like the common BWO. Rather, the male Baetis dun is a pale olive color, bordering on tan, while the female dun is decidedly yellow or "Sulphur" in color.

Sulphur dun

Furthermore, in our samples, female duns outnumbered males during the hatch by about two to one. In the air, both the male and female duns look "Light Cahill" in color. We do not know why the female duns outnumbered the males in our samples, but there are several possibilities.

It is known that some mayfly species can lay eggs without mating, and that early-season hatches often have higher proportions of males. Also, it is possible that the males, for some reason, exhibit greater mortality during hatching, or come off in sparser, less-noticed hatches during other times of the day.

Gustafson, who examined all three Paradise Valley spring creeks, has determined that in midsummer, mature nymphs of only three mayfly genera account for more than 95 percent of all mayflies in the streams--Baetis tricaudatus, Ephemerella inermis, and Callibaetis. There are also minor populations of Paraleptophibia debilis, Diphetor hageni, and Attenella margarita. In addition, the Armstrong's/DePuy's system (but not Nelson's) has a few Tricorythodes minutus.

Sulphur spinner

So why has the misidentification of this species taken place? Probably, amateur entomologists have made two classic mistakes. First, they based their insect identification on color, which is generally not an indicator of genus, let alone species. Don't take anything for granted. Second, they overlooked slight differences in the wings. The genus Centroptilum generally has a "spur" or costal angulation on its hind wing (in fact, the British refer to this genus as the "spur-wing"). But so does Baetis. The spur on the hind wing of the Centroptilum is hooked, but the spur on the Baetis is straight.

Furthermore, the Baetis has two intercalary veins (the small veins between the main veins) on its main wing; the Centroptilum has one vein. A good quality 8-power loupe or microscope is needed to make these distinctions.