Net-Spinning Caddis

By Rick Hafele and Dave Hughes

Unlike many other caddis species, net-spinning caddis don't build a case in which to live. Instead, they build a rough shelter of gravel and plant debris that they attach to the sides of rocks.

The term "net-spinner" derives from the spider-like web these caddis larvae weave at the front of their shelters. These silken nets strain the larvae's food from the currents. The need for current to bring them food means that nearly all net-spinners live in flowing waters (a few species occur along wave-washed shores of lakes).

Among the net-spinning caddis, four genera from the family Hydropsychidae are most important to Western fly fishers: Arctopsyche (Great Gray Spotted Sedge); Parapsyche (no common name); Hydropsycbe (Spotted Sedge); and Cheumatopsycbe (Little Sister Sedge).

Parapsycbe and Arctopsyche inhabit waters from small headwater streams to moderate-sized rivers with fast currents and large rocky bottoms.

The genera Hydropsycbe and Cheumatopsyche, on the other hand, prefer moderate- to large-sized streams with warmer temperatures, somewhat slower currents, and smaller substrate than the other two genera.

Like all caddis, net-spinners pass through four stages of development: egg, larva, pupa, and adult. Most species require one year to develop from egg to adult. A few species pass through two generations in a single year.

The exact emergence periods are difficult to predict because of the variety of species and habitats. In general, hatches are sparse in the spring. By June, activity increases. Often, the first large emergences occur during and just after the salmonfly hatch. The heaviest activity often occurs in July and August, but several species continue emerging into September and October.

Remember, hatches in the Rocky Mountain region will be two to three weeks later than hatches in the Pacific region. Small streams will have later hatches than large rivers.

The larvae of net-spinning caddis periodically crawl out of their shelters, let go, and drift downstream 40, 50, or even 100 feet. This activity occurs on a daily cycle, and peaks near sunrise and sunset. Entomologists call this "behavioral drift" and speculate that it functions to disperse insect populations, thereby relieving competition and allowing the colonization of underutilized areas.

For the fishermen, it means increased food for trout, making nymph fishing during periods of peak drift very effective.

Some hydropsychid larvae throw a twist into normal drift behavior. Instead of simply letting go of the substrate, they attach a silk thread to the bottom and lower themselves downstream on a "life line." In his book Caddisflies, Gary LaFontaine discusses his increased success fishing with net-spinning caddis larva patterns when he colored the last 18 inches of his leader white to suggest this silk anchor line of the natural.

Once the larvae are mature, they seal themselves inside their shelter and transform into pupae. The pupae remain sealed inside until ready to emerge into adults. The complete development of the pupae typically requires four to six weeks.

When ready to emerge, the pupae swim to the surface, which is perhaps the most vulnerable period of the insect's life cycle. Trout feed selectively on the rising pupae, and imitating them is one of the most effective methods to use during a caddis hatch.

Peak emergence activity occurs in the late morning or early afternoon during the spring and fall. In mid-summer heavy hatches occur in the late afternoon and evening.

Adult hydropsychids spend most of their time hiding on streamside vegetation. Mating occurs on the foliage, and unless a wind blows them over the water they are unavailable to fish.

Once egg laying begins, however, their vulnerability increases dramatically. Large swarms of gravid females congregate over the water from afternoon to late evening. To lay their eggs, they dive into the water and swim to the bottom, where they deposit strings of eggs on the substrate. Once egg laying is complete, they swim feebly back to the surface.

Such behavior makes them easy targets for feeding trout and an important stage for fly fishers to imitate. On streams like the Deschutes in Oregon or the Missouri and Big Hole in Montana, some of the fastest and most consistent fishing of the season occurs during the last hour of light when the hydropsychids lay their eggs.

Nymph fishing can be productive, but difficult. For caddis larva, I prefer a floating line and a 9-12 foot leader (shorter if I'm fishing small streams). Fish upstream with relatively short casts of 15 to 30 feet. Since the larvae inhabit riffles, that is where to fish the imitations.

Use a strike indicator to help spot subtle takes. Eddies behind rocks, current seams, and bottom depressions are all excellent areas for holding fish. The Net Builder is a decent imitation, and Zug Bugs also do well as a larval imitation.

Simple Soft Hackles are excellent patterns for imitating caddis pupae, as is La Fontaine's Sparkle Pupa . Selecting the proper size and color is a critical factor. But catching pupae is not as simple as catching larvae. During a heavy hatch, an aquarium net can effectively pluck the pupae from the currents. At other times, hold a kick screen in the current for several minutes. Unfortunately, the size and color of the adults do not accurately reflect that of the pupae.

Tackle and tactics similar to dry fly fishing work well for fishing pupa patterns. Fishing upstream, across stream, or downstream can all be effective. Fishing down-and-across with a slack line so the fly drifts naturally downstream, then rises to the surface just in front of a feeding fish is often deadly, and has proven successful on streams like the upper Sacramento in California. Strikes may be subtle or hard, but most will be near the surface and visible, so a strike indicator is rarely needed.

Once ovipositing females begin to appear, dry flies come into their own. Traditional patterns like the Elk Hair Caddis work well.

Since hydropsychid females dive underwater, wet flies such as Soft Hackles and the Diving Caddis can also be very effective when fished at various levels from the bottom to just under the surface. After laying eggs, the naturals drift with little movement, so a dead-drift presentation usually works best.

In the fading evening light it is often difficult to tell if fish are taking pupae or adults. When feeding on adults, however, surface rises will be much less vigorous than when taking pupae.

--Rick Hafele

A Rule for Fly Selection

My best evening fishing on the Deschutes came during a mid-summer emergence of net-spinning caddis. The naturals were thick in the air, and trout were coming up with splashy rises all around me. It was so dark that I couldn't quite tell what was going on. But it became evident, through the failure of my dry fly, that the trout were feeding on another stage of the insect.

I tied on a size 14 March Brown Spider. That's a Soft Hackle with orange thread, gold tinsel ribbing, and body of tan rabbit dubbing. In the water, it looked ragged but alive. I fished it on the swing through the splashing trout, and it was whacked it on nearly every cast. Some of those trout weighed more than three pounds and fought hard, using the strength of the current to fuel their runs. I never did find out if they were taking pupae rising for emergence or female adults swimming down to the bottom to lay their eggs, but in the midst of success the answer to that question seemed irrelevant.

Trout feeding on both rising and descending caddisflies often take them so near the top that the result is a splashy rise. It's easy to misinterpret this for a take to an adult on the surface. When trout are taking adults, a dry fly will catch fish. On the other hand, your dry will be ignored when they're taking pupae or egg-laying females.

So I have a rule with caddis, because so many of them are taken subsurface rather than on top: If I see caddis in the air, and scattered splashy rises, I try a dry first. But I switch quickly to a Soft Hackle or wet fly as soon as the first few trout refuse the floating fly.

--Dave Hughes

Rick Hafele is a professional entomologist and fly fishing writer living in Gresham, Oregon. His most recent book is Nymph Fishing Rivers and Streams. Rick's good friend Dave Hughes is fly fishing's most prolific author, with over 30 books to his credit including Trout Flies. Together they are the authors of Western Mayfly Hatches.