Understanding Stoneflies

Believe it or not, spring fly fishing is right around the corner. We know, we know—grabbing your fly rod and hitting the river might seem like a grim prospect to those of us getting buried in February snows, but some prime hatches are on the horizon and there’s no better time to get ready for the coming season with a brief entomology refresh. Cozy up to the fire and get comfortable. Today, we’re diving into all things stonefly! 

What is a Stonefly?

For the uninitiated, “stonefly” can refer to any one of about 2,000 species of aquatic insects that fall under the order Plecoptera. These species have a few traits in common: long antennae; two pairs of delicate, membranous wings; weak, chewing mouthparts; two tails (called cerci); and drab coloration that allows them to blend in with the river rocks, moss, and detritus where they spend most of their lifecycle. Their size varies significantly, ranging from just a few millimeters to over 60 mm (or about the length of a pinky finger), and can be an important characteristic to consider when choosing flies for your fly box.

This blog is all about the stonefly lifecycle and how to leverage that knowledge on the water. That said, it’s worth taking a moment to note that these bugs play a pivotal role in the ecosystems they inhabit. Though a female can lay up to 6,000 eggs, relatively few of these will survive all the way to adulthood—larger aquatic insects, fish, small mammals (such as turtles and otters), and waterfowl feast on stonefly nymphs year-round.

They’re also an excellent litmus test for the health of a freshwater ecosystem. They thrive in oxygen-rich environments where the water is cool and swift. When the water temperature rises or pollution enters the system, the stonefly population (among a few other key indicator species) will be the first to disappear.

We consider ourselves particularly lucky—here in Montana, rivers like the Madison, Big Hole, Yellowstone, Beaverhead, and Bitterroot come alive with stonefly hatches each year.

The Stonefly “Hatch”

The stonefly “hatch” might be better described as the stonefly “spawn”, as it refers to the time when they move from the nymph stage of the life cycle to full-fledged flying adults that return to the river to lay their eggs and die. The dries we use during this time, then, are imitations of adults fluttering on the surface of the water while laying eggs or spent adults. Nymphs, on the other hand, are present year-round and are, therefore, an important mainstay for fly fishermen planning to visit freestone rivers and streams.

Eggs are typically laid on the surface of a stream or river and eventually sink down to stick on river rocks or detritus. Depending on the species and environmental conditions, such as water temperature, it can take anywhere from a few weeks to several months for those eggs to hatch.

Once they do, the nymphal stage can last up to four years (again, it varies depending on the species). They prefer areas where gravel, cobble, or larger rocks offer some shelter from swift currents and will feed on all manner of algae, plant matter, and small aquatic insects.

Nymphs undergo a series of developmental stages called “instars”. With each new instar, the insect grows before shedding its exoskeleton. Unlike other species of aquatic insect, stoneflies do not have a pupae stage, instead emerging from their final molt as full-fledged adults.

During emergence, a nymph crawls from the water onto surrounding rocks or vegetation to complete this final molting. At this point, their lifespan is generally limited to a few frenzied days or weeks during which time their primary goal is to find a mate and reproduce.

Curiously enough, despite their fully developed wings, stoneflies are rather poor fliers. As a result, they end up remaining quite close to their stream or river of origin. Once females have laid their eggs, they generally end their lifecycle as spent adults—excellent snacks for the opportunistic trout.

Fishing with Stonefly Nymphs

Because the nymphal stage lasts for several years, they remain an important source of food to trout year-round—a fact we anglers can capitalize on if we use the right flies and techniques.

The most important thing to remember when nymphing with a stonefly pattern is that stonefly nymphs are bottom dwellers; you’ll need to get your fly down fast to deliver a believable presentation to wary trout.

Aside from crimping on some extra weight, there’s a few different techniques that can be helpful depending on where you’re fishing.

Casting upstream at the head of a riffle or run and allowing your fly to dead drift in the current can imitate a stonefly that’s been caught in the current. As a tip, we like to bend our stonefly patterns to make them look more natural. Tumbling down the river, a live Stonefly will try to roll up as it tumbles in an effort to get back to the bottom of the river as quickly as possible.

In deeper waters, consider fishing with an indicator. With your fly tapping and drifting across the bottom, picking up the more subtle takes can be challenging without one.

You might also consider using a dropper rig. Throw a stonefly nymph on the bottom and a dry fly on top. This can allow you to try more flies and appeal to both bottom and surface-feeding fish.

Common Species and Stonefly Patterns

While nymphs are present year-round, fishing with stonefly patterns (both nymphs and dries) tend to be especially effective in the spring and summer when mature stoneflies nymphs begin to emerge and adults are returning to the water to lay their eggs. In Montana, there’s a few key species to pay attention to:

• Skwala are smaller, olive-colored stoneflies and tend to be the first to emerge in late winter and early spring in western waterways like the Bitterroot River. (Pro-tip: once water temperatures reach about 40 degrees, you’ll see these guys start to emerge… Be ready to brave some potential whiteouts for some stellar fishing!) The Rogue Skwala, Rastaman Skwala, Water Walker Peacock, and Peacock Speckled Double Bead Stonefly are a few good patterns to try.
• Salmonflies are orange-tinted behemoths that start emerging in late spring and continue well into the summer. The Madison, Yellowstone, Gallatin, and Jefferson Rivers all offer some stellar feeding frenzies. Try a Rogue Foam Salmonfly, Chubby Chernobyl, Gridle Bug or Bitch Creek (nymph stage imitations), Morris Fluttering Stone, or a Kaufman’s Stimulator with rubber legs.

• Golden Stones are a mid-sized fly that come in mid-summer on waters like the Blackfoot River and Rock Creek. PK Goldens, El Camino Grillo Golden Stones, Demoes Golden Stones, Water Walker Golden, and Pat’s Rubberlegs (nymph stage imitation) are all good solid patterns.
• Yellow Sallies are small, delicate, and (you guessed it) yellow. They tend to emerge in June and July on the Yellowstone, Madison, and Jefferson Rivers and can be imitated with a Madam X, Yellow Stimulator or Iron Sally, Two Bit Sally, Juju Sally for nymph stage imitations.

 

Have any pro-tips to share? Questions to ask? Stories to tell? Let us know in the comments below!