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The Lesser Net-Spinning Caddis
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A few years ago I found myself standing knee deep in the Missouri River west of Helena, Montana. The Missouri produces great hatches and great dry fly fishing. As I stood quietly gazing at the water, dozens of feeding trout methodically dimpled the surface. I guessed that caddis were the main item on the trout’s menu, as the shoreline willows I had just pushed through were hopping with adults. Given the selective nature of these trout, however, I thought it would be good to know exactly what caddis was dancing around my face. My first guess was some species of Hydropsyche, but the adults seemed too small, so I picked up a couple fist-sized rocks to see if any pieces to the puzzle would reveal themselves. As I suspected some half inch long Hydropsyche larvae crawled across the stones like small inchworms. There was something else, too. The rocks had small silken tubes that twisted in all directions over their mottled gray surface. At first I thought these might be the tubes of midge larvae, but then out of the ends of several tubes crawled creamy light brown caddis larva. Looking at other rocks in the shallows I could see they too were covered with the twisted outlines of small silk tubes. Upon closer inspection many of the tubes were closed off with neatly developed pupae hidden inside. The abundance of the size 18 caddis adults on the willows now fit; the river was loaded with Psychomyiidae, a family of caddis little known to most anglers. So who and what exactly are the lesser net-spinning caddis? Actually there are three families of caddisflies that fall into this group: Philopotamidae (40 species), Polycentropodidae (70 species), and Psychomyiidae (17 species). Each family contains a diverse array of species in North America as noted, but only a few species are important to anglers. The attached table lists the nine most important species along with their general distribution in North America (E = Eastern, M = Midwestern, W = Western), emergence periods, and relative importance to fish and fishermen. Four "stars" indicate the species produces excellent hatches over a wide area and the angler would be wise to carry some patterns that match it. Three "stars" mean that the species is often important but is not as widely abundant as the four star species. Finally, two "stars" means it is an important species but is more restricted to certain habitats or geographic areas. Common names for these species are poorly established. Because they haven't been discussed in most angling literature, there is no historical precedent of common names. The common names I've listed are from Gary LaFontaine's book Caddisflies. Hopefully this will provide some consistency and less confusion then making up some of my own. That’s the “who” of the lesser net-spinning caddis, now how do you identify them from the other 1,000 species of caddisflies? This is a question not easily answered. First of all species identification is not even something the angler should shoot for. It requires adult male specimens, up-to-date regional keys, and a good microscope. Identification to genus is not impossible, but is beyond the space available in a short column such as this. A good key like An Introduction to the Aquatic Insects of North America, by Merritt and Cummins is needed. Family level identification is practical for most interested anglers. The larval stages are the easiest to identify to family and genus. Adult identification requires the use of a hand lens and more knowledge of anatomy. The characteristics listed in the attached table will help you identify the larval stages to family. The family Philopotamidae contains six species important to anglers: three in the East and Midwest and three in the West. Of the three eastern/midwestern species, Chimarra aterrima is most widespread, abundant and important. C. aterrima takes one year to complete its life cycle, with emergence concentrated in April and May. Its peak emergence often happens at the same time that the Hendrickson hatch (Ephemerella subvaria) is in full swing. As a result many anglers overlook the importance of this caddis. Larvae generally prefer the undersides of rocks in riffle areas. In the winter, however, larvae may migrate to deeper water, and in the summer during warm weather they are commonly found on the submerged roots of streamside grasses. It is the activity of the adults that make this species so valuable to the angler. Females return in large concentrations in the late afternoon or early evening to lay their eggs on the water's surface. Many drown and most drift long distances on the surface creating the perfect situation for great dry fly action. Adults are in the 18 to 20 hook size range, so be prepared with small flies and light tippets. Earlier in the day, usually around noon, the pupae are active and are also taken readily by waiting trout. Into this mix add a good Hendrickson hatch and some interesting fishing will be at hand. Of the three western species, Dolophilodes aequalis is most important in Montana and small Rocky Mountain streams and rivers. The two western species of Wormaldia produce some excellent hatches, with W. anilla more abundant in small streams of the Pacific Northwest and W. gabriella important in larger rivers throughout the West. W. anilla appears to produce two broods a year, with the first emerging in April and May and the second emerging in the fall, September through November. The Eastern species, Dolophilodes distinctus, also has a split emergence; first in mid-summer then again in mid-winter. The winter hatch, plus the fact that the winter hatching females are wingless, make this species unique. The larvae of Polycentropodidae are sometimes called trumpet-net caddis because of the trumpet shaped net, open at both ends, most species construct. Of the three listed species, two are widely distributed across North America and important to anglers in many regions of the country. Nyctiophylax moestus prefers pools, slow runs or the gentle currents along the edges of streams to build its silken retreat. The larvae are predators, and they use the threads of their retreat to detect prey much like a spider detects food in its web. When a drifting nymph bumps into the caddis larva's net and vibrates the threads, the larva rushes out to grab its dinner. Pupae emerge in the late afternoon by swimming up to the surface in flats and slow runs. Adults are active at about the same time of day, laying eggs by swimming or crawling down objects to the stream bottom. The combination of emerging pupae and egg laying adults can create an excellent rise of fish, but it can be tricky to tell just which stage they are taking. If you're not having luck with a dry fly, fish a pupa or submerged adult pattern with a slow downstream swing. Polycentropus cinereus has very similar habits as N. moestus. The main difference is that P. cinereus prefers clear, cold water streams or the wave washed shores of cold lakes. Most of the activity for this species occurs in the evening when pupa emerge in the surface film, and females return to lay eggs by diving underwater and swimming to the bottom. It is again important to determine which stage the fish are most actively feeding on. Two species of Psychomyiidae are important to anglers. The first, Lype diversa, occurs only in Midwest and Eastern states where it produces good hatches on small woodland streams. The larvae almost always live on sunken logs and branches, where they lie in a shallow groove in the wood with a roof of silk netting. This simple silk tube is further camouflaged with small pieces of debris. As a result most anglers overlook the larval retreats and do not realize how abundant they really are. Adults are active in the afternoon and evening shortly after the pupa swim to the surface for emergence. Adult activity is usually easy to spot as fish generally respond well to the pupa and adults. Psychomyia flavida is by far the most important species of this family. It is widespread, occurring across the entire country, one of the most abundant species of stream dwelling caddisflies, and prefers cold, swift moving rivers where trout also find ideal conditions. Only one thing diminishes the importance of this small caddis; the heaviest emergence and egg laying activity tends to occur after dark. When dark clouds cover the sun, however, emergence and egg laying may take place through most of the day creating some excellent fishing opportunities. A quick flurry of activity can also occur when the sky darkens just before a summer thunderstorm hits. The pupae and adults are small, so look carefully for adult activity and be prepared to use size 18 or 20 imitations. This is the species I found so numerous along the willow-lined shores of the Missouri. Fishing tactics for the lesser net-spinning caddis are similar to the techniques used for most good caddis hatches. Larval populations can be dense, and while they spend most of their time hidden inside their silk tunnels, they do occur in stream drift, especially the larvae of philopotamids and pschomyiids, which live in riffles and runs. For larval imitations use a size 16 or 18 brown to light tan larva pattern, and fish it dead drift right off the bottom. I prefer to use two nymphs for this fishing, combining the caddis pattern with a stonefly or mayfly nymph, depending on what other insects are abundant. The pupae and adults create the most exciting fishing. As previously mentioned the naturals are small, 18's or 20's, so have some small patterns ready. Small soft hackles or sparkle pupa work well for pupa imitations. Colors vary with species and range from black (Chimarra and Lype), to brown (Dolophilodes and Psychomyia), to olive (Wormaldia), to light yellow (Nyctiophylax). Fish the pupa patterns with a slow Leisenring lift or a simple down and across dead drift near the surface. Adults both sit on the surface and dive underwater to lay their eggs. Thus, try either a standard dry fly approach – fishing a dry fly dead drift on the surface – or a wet fly approach by fishing your dry fly subsurface. The rise forms to the small net-spinning caddis are not generally as vigorous or splashy as they are during other caddis hatches, so watch the surface carefully for signs of feeding fish. It is also a good idea to look for adults on streamside trees or shrubs. Though small, the adults will be quite obvious during a prolific hatch as they run and hop from leaf-to-leaf, or leaf-to-trout.
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Net-spinning caddisflies – so called because of the spider-like silk webs they spin for catching food – are perhaps the most important caddisflies to anglers across the continent. The dominant species of net-spinning caddis belong to the family Hydropsychidae and two main genera: Hydropsyche and Cheumatopsyche. A wide variety of species in these two genera produce exceptional hatches from Oregon to Maine, in large rivers to small mountain streams. While these hatches are well known and documented, there are many other species of net-spinning caddis that produce wonderful hatches and yet have remained relatively unknown. I refer to them as the “lesser” net-spinning caddis because of the minimal attention they have received in angling literature. You will find fish just as interested in their imitations during good hatches as any of the more famous caddis hatches.