tactics

Weedy Water Caddis

By Jeff Morgan

This slender caddis doesn't occur on all streams. And when it's present, most anglers don't know about it. That's not a mistake that trout make, however.

or most Western fly fishers, the little western weedy-water caddis--Amiocentrus aspilus--is of marginal importance. The species is only located in some Western streams, preferring slower water with a higher degree of decaying vegetative matter than their cousins the grannoms (Brachycentrus americanus).

This habitat requirement relegates this caddis species to plunge-pool streams west of the Cascades as well as spring creeks throughout the West.

Fortunately for fly anglers, when the weedy water caddis is present, it is usually in sufficient densities to be important to trout. In one stream in the northern Oregon Cascades, I located over 200 mature larvae per square foot of bottom in a leaf-strewn side eddy. Dr. N. H. Anderson (emeritus professor of aquatic entomology at Oregon State University) once located over 700 larvae per square foot on Oregon's Metolius River. When insects of this size are this dense, trout take notice.

Larva

The larvae resemble grannoms in color--bright green body, dark head--but with the physical dimensions of an anorexic supermodel: very slim and long.

The case material is usually composed of a wood-plant material woven into a chimney-like case. The easiest way to distinguish between a grannom and a weedy-water caddis is to look at the case. A grannom's case is a tapered box with four distinct sides, while the weedy-water caddis's case is a long round cone. A mature larva usually tops off between 8-10 millimeters in length.

As mentioned before, the larvae prefer water with plenty of decaying vegetative matter, and when present, can often be found crawling around on the shallow margins of pools and eddies where leaf matter is most abundant. The slow-water preference of this caddis minimizes the importance of the larvae to trout. However, in times of spate, dislodged larvae would have a difficult time regaining contact with the substrate, due to the light plant material with which they build their cases.

When imitating the larvae, first be sure to find naturals in the area. Many streams don't even have a population of these insects, so don't imitate them in vain. If you ascertain that the cased larvae are present, drift an imitation near the bottom. A good simple pattern is a beadhead Amiocentrus Larva, which is tied on a long-shank nymph hook with a pheasant tail body, a wrap of bright green dubbing for a collar, three black deer-hair legs cut short, and a small black beadhead. If you capture a natural larva, you can see that this pattern looks very similar to the natural.

The optimal times to fish this pattern, and all other cased caddis larvae, are at dusk, when there is plenty of foot traffic in the river upstream of where you are fishing, or during a spate.

Pupae and Emergence

Prior to emergence, the larvae congregate, anchor the posterior of their cases to the substrate, seal the tops of cases, and begin to pupate. Since they usually pupate in slow currents, their cases extend at a 90-degree angle to the bottom. Thus, an attentive angler can easily see, without having to pick up rocks or use a kick screen, whether a major emergence has already occurred or if they are still pupating: when some cases are open (indicating an emerged pupa) and others are still closed (indicating an immature pupa) you have likely arrived in the middle of an emergence period. Immediately downstream of this "pupation bed," trout should be very receptive to a pupa imitation. The pupae of this species are often 7-11 millimeters in length, with a green abdomen and a darker thorax.

Save your pupae imitations for when you locate a pupation bed or when you see the adults coming off. Emergences on particular streams are often concentrated to a short period, so trout will feed heavily on this insect during a hatch. Additionally, Amiocentrus pupae take their sweet time getting to the surface, and trout will grow fat on this extended drift.

The pupae of these insects look very similar to those of grannom (green body, dark gray wing pads), and similar patterns, such as green Sparkle Pupas, Z Wing Caddis, and olive Soft Hackles, will fool the fish.

Adults

Adults of this species are rather ephemeral for caddis, and will rarely live longer than several days. The mature insects are often a grayish-brown in color and range in size from 7-11 millimeters. They generally emerge in mid-summer.

Though the larvae are quite recognizable, it's difficult to identify an adult when you're on the water: they just look like all the other caddis adults.

The short-lived nature of the adult insect makes them of marginal importance to the fish, except when the females return to the water to lay eggs. Females sometimes deposit eggs on the surface (like grannoms), but are more likely to dive/crawl underwater to affix their eggs to the substrate (like some species of saddle-case caddis or spotted caddis).

Because the adult stage of the weedy water caddis is very short, you probably won't fish imitations of them randomly. The adult has black wings, with a green or green-tinted/black body, and are about a size 14-16.

My favorite pattern is a CDC Caddis with a black wing and peacock body. The CDC style offers a great silhouette, and the fly rides right in the surface film: perfect for the flat water that this insect likes so much.

Other good patterns are a black Elk Hair Caddis, Tent Wing Caddis, or Parachute Caddis. Some anglers like to use dark Soft Hackles during the egg-laying period to imitate sunken or diving females.

Jeff Morgan has written many articles for Westfly, mostly on entomology and fly tying. He is the author of An Angler's Guide to the Oregon Cascades and Small Stream Fly Fishing. Jeff is currently a graduate student at Stanford University, where he is finishing his PhD in History.

Uploaded 04/15/2001.

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