Green Rock Work Vs. Net-Spinning Caddis
The green rock worm nymph is one of my all time favorite nymph patterns. I have found it effective every month of the year on streams throughout the West and beyond. But why? While green rock worm caddisflies (family Rhyacophilidae; genus Rhyacophila sp.) are common in many trout streams across North America, they aren’t found everywhere in large numbers and their abundance tends to vary seasonally. Thus the effectiveness of this fly pattern seems to go beyond its obvious imitation of Rhyacophila larvae. I believe the answer lies in the often overlapping characteristics of Rhyacophila with another, often more abundant stream dwelling caddisfly, the net-spinning caddis or spotted sedge, which belongs to the family Hydropsychidae. A close look at both may shed some light on these two important caddis and how they can be easily confused with one another.
Green Rock Worm or Green Sedge
All caddis in this group belong to a single genus, Rhyacophila, of the family Rhyacophilidae. With over 125 species in North America, Rhyacophila is the most diverse genus of all the caddisflies. Such species diversity accounts for its wide distribution with different species spread across the continent. But the habitat used by Rhyacophila is limited to quick, cold streams, with the best populations occurring in riffles of mountain streams. Their love of cold mountain streams doesn’t eliminate them from low elevation rivers, but their presence and importance is greatest where streams tumble down hills rather than meander across valleys.
The common name, green sedge or green rock worm, effectively communicates the overall look of this caddisfly. Larvae come in various shades of green, but most have bright green bodies that stand out quite readily on a brown or gray stream bottom rock. These larvae are also known as “free-living” caddis because the larvae do not cover themselves with any type of case, as nearly all other caddis larvae do. As a result green rock worm larvae are unprotected from feeding trout if they get whisked off the bottom and into the water column. Larvae get fairly large. Mature larvae may be one half to three quarters of an inch long, and thus matched with size 12 or even 10 hooks.
When ready to pupate, larvae crawl into a crack between some cobble in a riffle and cover themselves with small pieces of gravel, forming a rough but sturdy shelter to hide in. Here they remain for four to six weeks while the larvae molt into pupae and the pupae mature. Mature pupae cut out of the rough shelter and then swim for the water’s surface where the adult green sedge makes its escape, unless of course a trout intercepts the pupa on its way up or grabs the adult off the surface before it takes to the air, both frequent occurrences during good green sedge hatches.
Adults are a nice mouthful for a hungry trout as most species are closely matched with a size 12 dry fly. Peak emergence of adults in western streams is generally split between spring and fall. Spring hatches can begin as early as mid March but usually peak from mid April to mid May. Fall hatches can be good from mid September until late October. Adults are active during the afternoon when females dive underwater and swim to the stream bottom to lay their eggs. This underwater activity can be important to imitate when trout at feeding on adults.
Net-spinning Caddis
The dominant family of net-spinning caddis is Hydropsychidae. Within this family two genera are of particular importance: Hydropsyche (43 species in North America) and Cheumatopsyche (44 species in North America). Both of these genera are widespread across the country and are consistently one of the most abundant caddisflies in streams.
Larvae of this group feed by constructing spider-like webs of silk on the sides of cobble sized rocks in moderate to fast riffles. The webs effectively strain food like algae, small crustaceans, zooplankton, and small insects from the water column. Reaches below dams often have huge numbers of net-spinning caddis since plankton and zooplankton blooms in reservoirs above the dam spill downstream creating an abundance of food for these filter feeders.
The color of the larvae of many species is brown or dark olive-brown, but many other species are olive to bright green. The green colored larvae of net-spinning caddis are so close in size and shape to green rock worm larvae that the same nymph pattern does a fine job of imitating them. This is why I think the green rock worm nymph pattern is so effective in so many streams throughout the year. When and where green rock worms are sparse or absent, the green colored net-spinning larvae are often abundant.
Net-spinning caddis pupate in rough shelters on the sides or undersides of rocks in riffle areas very similar to green rock worms. After four to five weeks of pupation the pupae cut free and swim to the surface where the adults emerge. Also like green rock worms, fertilized female adults dive underwater and swim to the stream bottom to lay eggs. Summer is the time of year for some of the best hatches of net-spinning caddis with peak activity from mid June to mid August. The egg laying females are most active at dusk.
Because of their behavior, net-spinning caddis are readily eaten by trout as drifting larvae, rising pupae, and adults (both on the surface and below). Fly patterns for net-spinning caddis should usually be tied in sizes 14 to 16, about one or two hook sizes smaller than green rock worms patterns.
Telling Green Rock Worms from Net-spinning Caddis?
Given the similar size, shape, color, and even habitat used by net-spinning caddis and green rock worms, what is the best way to tell the two apart? For the larval and pupal stages the answer is simple - gills. Net-spinning caddis have them and most green rock worms don’t. Let’s look at the larvae first.
All species of Hydropsychidae larvae have well developed tufts of filament-like gills along the underside of their abdomens. In contrast most species of Rhyacophilidae lack gills and those that do have them they do not occur along the underside of the abdomen. The same is true for the pupae. Hydropsychidae pupae clearly show gills along the underside of their abdomens while Rhyacophilidae pupae rarely show any gills. As a result the presence and location of gills makes for a quick and simple way to tell these two groups of caddis apart in the larval and pupal stages. Telling the adults apart is a little more difficult.
Probably the quickest way to tell net-spinning caddis and green rock worm adults apart is by size and the time of year. Green rock worms are clearly larger and emerge in the spring and fall; net-spinning adults are smaller and most emerge during the summer. Other than size there are difference in wing color patterns (green rock worms tend to have a salt & pepper like color pattern, while net-spinning caddis wings are more uniform in color for some species). The most reliable difference is also one of the hardest to see. It’s a difference in one of the mouthpart structures called the labial palps. Labial palps are like slender segmented fingers attached to the lower part of the mouth. The number of segments and their shape are often used to identify different types of caddisflies. In net-spinning caddis adults the terminal segment of the labial palps is long (as long as the other four segments combined) and flexible, while in green rock worms the terminal segment is not unusually long. Seeing the labial palps isn’t that hard if you know what you are looking for and you have a small hand lens handy.
Green rock worm or net-spinning caddis? At the end of the day it really doesn’t matter much. As long as you match the size and color closely and fish your fly correctly, the trout won’t care if you think you are imitating a green rock worm when really it’s net-spinning caddis. Which brings us full circle back to my green rock worm nymph pattern (or any other green rock worm pattern). You can use it wherever green rock worm OR net-spinning caddis larvae are present, which is just about every month of the year in nearly every trout stream in North America.
Happy casts!