Caddis are commanding members of the aquatic insect community on all of the trout waters in and around Yellowstone Country. From April through October, there isn’t a day that goes by without some sort of Caddis activity. Of the dozen or more different types of Caddis that produce good fishing in Yellowstone Country, the genus Hydropsyche is the most important. From May through August, Hydropsyche Caddis consistently produce more quality fishing situations than all of the other Caddis combined.
There are four unique species of Hydropsyche found in Yellowstone Country, and luckily there’s no direct benefit to differentiating between them for the fly angler. All four species have tan bodies, wings that range from tan to speckled brown, and vary in size from 14 to 16.
Next to Pale Morning Dun mayflies, there is not a more ubiquitous insect in Yellowstone Country trout waters than the Hydropsyche Caddis. If you are fishing moving water anywhere in our region you can find Hydropsyche in the larvae, pupa, or adult stage.
Hydropsyche larvae are net spinners who build unsophisticated shelters out of silk that double as a food catching structure. In his groundbreaking work Caddisflies, Gary LaFontaine reports observing Hydropsyche larvae using a silk anchor line to hang in the current as a method of migration from rock to rock. LaFontaine even went so far as to experiment with white-colored tippet to imitate this anchor line.
When Hydropsyche larvae begin metamorphosis and start the transformation to pupa they seal their rudimentary case with silk and remain entrenched for 2 to 3 weeks. The adult Caddis slowly grows within the pupal cuticle until it reaches maturity. Once mature, the insect chews its way out of the sealed case and begins it’s emergence.
Emerging Caddis pupae are, most simply, a fully formed adult draped in a thin pupal cuticle filled with gas bubbles. Hydropsyche pupae ride the current near the bottom for some time before rising through the water column. Once they reach the surface, pupae battle their way through the meniscus, and struggle to free themselves from the pupal shuck. This is an epic struggle that can take several minutes, and often times it’s unsuccessful. Large numbers of crippled and stillborn pupae can be found during emergences.
The best emergences of Hydropsyche Caddis occur from mid-June to mid-August in Yellowstone Country. All of our major fisheries have robust populations of Hydropsyche, and fishing the evening emergence and morning egg-laying flights are ingrained in the rich angling history here.
Adult Hydropsyche Caddis spend 2 -3 days resting after emergence before they begin to form mating swarms. Mating flights generally occur well above the water, or around streamside vegetation. It’s not uncommon to see dense clouds of Caddis surrounding riparian willows as the sun gets low in the Western sky and the heat of the day subsides. These massive mating flights generally provide nothing more than a distraction to anglers as the insects seldom touch the water.
Egg-laying females, on the other hand, can be very important. Female Hydropsyche return to the water after mating, and dive to the bottom to lay their eggs. After releasing their eggs, the females drift in the current and slowly ascend back to the surface. There, the egg-layers fight once again to break through the meniscus and make an apparent re-emergence. Spent, egg-laying females then helplessly ride the surface currents. From the time egg-laying Hydropsyche re-enter the water to the time they re-emerge, opportunities abound for trout to prey on them. Egg-laying activity peaks in the morning and evening hours, though females can be seen sporadically throughout the day.