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Fishing Freshwater Isopods

“Cress bugs” or “sow bugs” ring trout's dinner bells 365 days a year.

Fishing Freshwater Isopods

If you fish in a stream or river with lots of weeds, there are also likely freshwater isopods. These little crustaceans are consistently available to the trout through all seasons, and they’re one reason why trout in tailwaters and spring creeks are often so large. (Eric Naguski photo)

This article was originally titled "Freshwater Isopods" in the Rising Tides section of the Aug-Sept 2021 issue of Fly Fisherman magazine.

The modestly sized brown trout held in about 6 inches of water, positioned in the current of the eddy so it was facing me, even though I was fishing upstream while stalking the edges of this central Pennsylvania limestone spring creek.

I could see the fish plainly in the bright sun and clear water. Luckily I didn’t spook this one as I had the previous fish I came across. It was a hot midsummer day, and there were only a few leftover Trico spinners floating dead along the edges and eddies of the stream. My last cast had sent the previous fish darting for the cover of deeper water.

I watched the fish for a few minutes. He wasn’t a giant, but still a respectable fish for this stream. Every minute or so, the fish moved forward, stuck its head into a small patch of submerged aquatic vegetation, then dropped back to its original position to feed. I immediately recognized that the fish was rooting around in the vegetation and knocking cress bugs, as we call them here in Pennsylvania, loose from their home in the weeds. The fish was creating its own “hatch.” After “shaking the branches” a few times, the trout looked down at a softball-sized rock and ate something from its surface.

I gently cast my lightly weighted size 14 cress bug pattern just upcurrent of the trout, directly over the vegetation the trout had been rooting around in. I plainly saw the fly drift downstream, then hang up on the bottom about a foot in front of the fish. I gently lifted the rod tip, intending to wiggle the hook free from the bottom, and the slight movement caused the fish to pounce. Pretty satisfied with the whole scenario, I brought the fish to hand, and admired it momentarily before it darted off into the current.

This is the trout hunting that I love, and it’s what keeps me on the water in the heat of midsummer and during the freezing snow squalls of winter. Living and fishing in spring creek country affords year-round opportunities.

Aquatic Crustaceans

Freshwater isopods (locally known as cress bugs or sow bugs) are some of the most important food sources for trout in spring creeks and tailwaters. They make up a large portion of the biomass, and are available to the trout every minute of every hour of every day of the year, and the fish know it.

Freshwater Isopods, crustaceans, sow bugs, cress bugs
Freshwater isopods—known locally as cress bug or sow bugs—are crustaceans that form a large portion of the biomass in spring creeks and tailwaters across North America. They are flattened and oval, and are most often colored to match the stream bottom. (Eric Naguski photo)

Aquatic isopods, favorite foods of trout and other freshwater gamefish, are crustaceans in the order Isopoda.  They are related to the terrestrial sow bugs, pill bugs, and roly-polies you find under rocks in your garden, and look much the same.

Many of the aquatic forms live in subterranean habitats (caves and springs) but there are two genera in the family Asellidae that are of interest to trout anglers. Those genera are Caecidotea species and Lirceus species. Both are found in the streams of the Eastern United States, and the genus Caecidotea is more prevalent in the West.

Morphologically, species within the two genera are very similarly shaped. But in general, Caecidotea tend to be slightly more slender than the more ovoid Lirceus. Sow bugs seem to be the lesser known of the aquatic crustaceans and don’t seem to get quite as much attention as Amphipods (scuds), but they play an important ecosystem role in many spring creeks and tailwater systems across North America.

Sow bugs feed on decaying organic matter, most often plant material, and are almost always found in streams with large amounts of aquatic vegetation. Sow bugs breed year round, and the female carries the fertilized eggs and then holds the newly hatched young isopods in a pouch-like structure called a marsupium for 20 to 30 days. This process is called direct development. As sow bugs mature they will have at least 15 instars (molts), growing larger with each molt. They become adults after five to eight instars. The fact that sow bugs reproduce year round makes them especially important to trout. Unlike most mayflies, caddisflies, and stoneflies, there is never a peak time or a corresponding low time in terms of availability to trout.

The coloration of sow bugs is variable but is most often an olive-gray with a median dorsal “stripe” of darker pigmentation. This stripe is especially conspicuous in Lirceus.


In some trout waters, the abundance of sow bugs is truly astounding. In one study on the Bighorn River, Montana, researchers found as many as 7,213 sow bugs per square meter. It’s no wonder the Ray Charles Sow Bug was created there and is still effective today. All the weedy Western tailwaters have cress bugs, as do Western spring creeks like the Paradise Valley spring creeks in Montana, and California spring creeks like Hat Creek, Hot Creek, the McCloud River, and the Fall River.

Just a few of the trout waters here in Pennsylvania with good sow bug populations include all the limestone spring creeks of Cumberland Valley (Letort, Big Spring, Yellow Breeches, and others), Spring Creek in Centre County, and Spruce Creek in Huntingdon County.

They are also in the spring creeks of the Shenandoah Valley in Virginia (Mossy Creek and Beaver Creek), in the West Branch of the Delaware River, and in many of the TVA tailwaters in the South including the South Holston, the Watauga, and the Toccoa rivers. This list is not inclusive, as cress bugs are incredibly widespread, important food sources for trout in almost all weedy environments.

Flies and Techniques

Sow bugs are dorso-ventrally flattened and either shaped like a football (ovoid) or not quite as wide and shaped more like a Popsicle stick, rounded on the ends. The shape of sow bugs dictates the design of the flies.

Cumberland Valley legend Ed Shenk was the first fly fisher to write about imitating freshwater isopods in his article  “The Cress Bug” in the January 1972 issue of Fly Fisherman. Most of the subsequent sow bug patterns developed in North America have Shenk’s Cress Bug DNA in them. Three more modern patterns popular on the spring creeks of the East are Jake Villwock’s SYE Sow Bug (Simple Yet Effective), and beadhead Jiggy SYE. Another effective pattern is my own HG (Hot Glue) Cress Bug. I am confident you can use any one of these three patterns and be successful imitating sow bugs anywhere.

Freshwater Isopods, crustaceans, sow bugs, cress bugs
Shenk’s Cress Bug, first described in the January 1972 issue of Fly Fisherman, is the grandfather of most cress bug imitations tied today, and it still catches fish. (Eric Naguski photo)

There are many other great sow bug patterns out there—George Daniel’s Joe’s Fur Cress Bug is another that comes to mind. Ask any experienced spring creek angler, and they will either have their own pattern, or tell you which preexisting patterns is their favorite.

Your fly color should be in the olive-gray range. Get to know the sow bugs in your local waters to customize the colors of your patterns to match what is common. If you are traveling to fish, and think you may run into some sow bugs, call a local shop. They can definitely point you in the right direction. But as a general rule, I like to have a few patterns in olive, olive-gray, olive-tan, and a few that are primarily gray with a light amount of olive and tan. One of the most important characteristics is the dark dorsal median stripe that is usually a darker olive-brown. Sizes range from #10 to #18, but in general #14 and #16 are the best sizes to start with.

An important consideration for a good sow bug pattern is the weight. Even if it looks real to you, a pattern that is too heavily weighted will not tumble along the bottom like the real sow bug. I have spent probably way too much time just watching what happens to sow bugs when something causes them to lose their grip, and they are picked up by the current. They tumble through the water column, slowly sinking to the bottom. Sow bugs that are walking across the stream bottom and caught by the current just flip over and over like a rolling car until they can regain a hold on the bottom or some underwater vegetation or a rock.

You want enough weight in your fly to get it near the bottom, but not so much weight that it doesn’t move naturally. If you need extra weight, add split-shot.

I recommend fishing sow bugs with 4X to 6X fluorocarbon tippet. Most of our limestone streams are exceptionally clear, and the less the trout see of your tippet the better.

You can also fish sow bugs as part of a dry/dropper rig—there is no denying this is an effective way to fish this fly. The dry-fly choice should be dictated by the time of year. In the winter, a size 14 black stonefly pattern is a good choice. In the spring, caddisflies become more relevant. As you move into summer, a beetle, hopper, or cricket makes more sense.

Sow bugs can also be fished as you would any other nymph, under an indicator or as part of a tight-line nymphing rig. This is especially effective when conditions prevent you from sight fishing the fly to a specific fish. A drop-shot rig positions the heavy split-shot at the bottom of the rig, with the fly on a dropper above it to allow adequate movement of the sow bug pattern during the presentation. Another option for a tight-line rig is to use a heavily weighted bottom fly to get the sow bug into the correct position in the water column. Either rig can also be used with an indicator.

Sight Fishing

If you really want to up your game, try sight fishing a single sow bug to a specific fish—it’s a rewarding and entirely visual experience that paints everlasting memories in your mind. No one forgets the moment when a wild fish feels the sting of the hook and goes berserk in the confines of a small spring creek, and a quiet pastoral setting suddenly becomes a venue for mayhem.

Freshwater Isopods, crustaceans, sow bugs, cress bugs
Sight fishing to a feeding trout with a single sow bug is a rewarding and entirely visual freshwater hunting experience. If you want to add an indicator, use a sparse yarn-type indicator so it lands softly without the “plop” that may spook fish feeding in shallow water. Even with an indicator, try to watch the fish, not the indicator, as trout often take the fly without moving the indicator. (Eric Naguski photo)

A good sow bug pattern can make those kinds of memories happen. You can sight-fish a single sow bug pattern without a dry or indicator. A soft indicator such as a Dorsey indicator or New Zealand Strike Indicator is much more effective in these situations because they land softly. The last thing you want to do is plop a giant foam or plastic indicator on top of a pressured wild fish.

If you are sight fishing with a dry/dropper or indicator rig, make sure that you watch the fish, not the indicator or dry. The indicator is there primarily to give you an idea of where your sow bug is. You should be watching the fly, or at least the fish you’re after.

I often go without the indicator and use a single sow bug with a small split-shot about 6 inches above the fly. This technique is not for everyone, and often a dry/dropper or indicator rig makes things slightly less difficult. None of this is easy fishing, but I prefer the stealth of casting to single trout using just a single sow bug and a small split-shot. It is about as technical as you can get, but the payoff when you are successful is amazing.

I cannot tell you how many times I have watched a fish eat a sow bug without moving the indicator. On a day with good lighting, you can have a clear view of the fish’s reaction to your fly, and when it all comes together and you can feed your sow bug to a big wild trout, the experience is like no other.

When you are able to locate a feeding fish, you will find the most success by presenting your offering fly first. By this I mean that the fly should reach the fish first, before any other portion of your line, leader, or tippet. The best way to do this is with a downstream presentation. But to be in position to present the fly in this manner, you will have to be upstream of the fish. This puts you in the precarious position of possibly spooking the trout.

Freshwater Isopods, crustaceans, sow bugs, cress bugs, rainbow trout
When it all comes together and you can feed your sow bug to a big wild trout, the experience is like no other. (Photo courtesy Eric Naguski)

I recommend keeping your profile as low as possible, and making the cast from the side and from a distance that keeps you out of the fish’s line of sight. Place the fly a distance above the fish that allows the fly adequate time to sink to the bottom. You want the fly to be tumbling along the bottom naturally, just as it comes into the trout’s feeding lane.

Measure the distance to the fish with one or two false casts, add a couple feet to that distance, and on the delivery, stop the rod abruptly during the forward stroke, so the fly lands above the fish with a little slack in the leader. This will help avoid drag during your presentation.

If you cannot get into position to present the fly downstream to the fish, and you must approach from downstream, you’ll have to make extra effort to make sure you don’t line the fish with your cast. Angle the cast upstream, placing the fly and leader far enough upstream of the fish to allow time for your fly to sink.

If you cannot get into position to make an angled upstream cast, and you must cast from directly downstream, use a curve cast to place the fly directly above the fish and the rest of your leader and fly line off to the side of the fish, reducing the chances of spooking your target.

Even from downstream, stay low when you approach the trout, and make as few false casts as possible. The sight-fishing game can be really addictive. Once you meet with a little success, you may just keep coming back for more. And since sow bugs are often found in trout streams that are fishable year-round with relatively consistent conditions no matter what the time of year, you will have plenty of opportunities to try and feed your best sow bug imitation to what could be the trout of a lifetime.

Fly Recipes

Freshwater Isopods, crustaceans, sow bugs, cress bugs, fly recipes, fly tying, fly fishing
(Eric Naguski photo)

Jiggy SYE (left)
(Jake Villwock)
HOOK: #12-18 Tiemco 450BL.
BEAD: 2.8 mm slotted tungsten bead, mottled brown.
THREAD: Gray 8/0 UNI-Thread.
WEIGHT: 0.025" lead wire.
DUBBING: Sow bug Sow-Scud Dubbing.

HG CRESS BUG (middle)
(Eric Naguski)
HOOK: #12-18 Tiemco 230BL.
THREAD: Brown Veevus 12/0.
WEIGHT: 0.015" lead wire.
DORSAL STRIPE: Dark brown Wee Wool.
BODY: Wapsi Smokey Olive Sow Scud Dubbing.
SHELL: Hot glue trimmed to shape.

SYE Sow Bug (right)
(Jake Villwock )
HOOK: Orvis Tactical Wide Gap
UNDERBODY: Lead wire 0.025
THREAD: UNI Thread – red or gray
BODY: Sow-scud dubbing

Freshwater Isopods, crustaceans, sow bugs, cress bugs, book, Eric Naguski

Eric A. Naguski has a Bachelor of Science in Biology from Millersville University, with a focus on aquatic entomology and ecology. He is the author of Favorite Flies for Pennsylvania: 50 Essential Patterns from Local Experts (Stackpole Books, 2021). He is a guide and owner of Riseforms Fly Fishing in Boiling Springs, Pennsylvania and serves on the boards of Cumberland Valley Trout Unlimited and the Pennsylvania Fly Fishing Museum Association.

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