By Son Tao

It’s Hopper Season—the time of year when grasshoppers, crickets, katydids, and beetles seem to be everywhere, especially near the water’s edge. If you’re a fly angler, you know this is prime time. Terrestrials start showing up in trout diets, and the explosive surface takes are unforgettable. But have you ever noticed some insects seem to jump or fall into the water for no obvious reason?

Turns out, not all of them are careless. Some are being controlled.

Meet Spinochordodes tellinii, a parasitic hairworm that turns ordinary grasshoppers into unwitting swimmers. Its goal? To manipulate its host’s brain and send it straight into the drink.

The Underwater Beginning

The life cycle of this hairworm begins in water, where its microscopic larvae float, waiting to be consumed by an unsuspecting terrestrial insect. This can happen when a grasshopper drinks contaminated water or eats another small organism that’s carrying the parasite.

Inside the insect, the hairworm grows silently and stealthily. Coiled tightly within the host’s body cavity, it can stretch to several times the host's length, yet remain unnoticed. To the fly fisher's eye, that hopper on the bank still looks like a juicy, twitchy meal.

When the Water Beckons

But at some point, the worm matures, and its mission changes. It can’t reproduce inside the host. It needs water.

That’s when it starts to manipulate its host’s behavior. Using chemical signals, the hairworm interferes with the insect’s central nervous system, altering its natural aversion to water. What was once a cautious grasshopper now starts wandering toward streams and ponds. Eventually, it jumps in—or falls in—almost like it was drawn by some unseen force.

As anglers, we see this behavior on the water and chalk it up to clumsiness or wind. But some of those land-based insects are the victims of parasitic mind control.

The Worm Emerges

Once the host hits the water, the hairworm makes its dramatic exit. Long, thread-like, and often startling to witness, it wriggles out of the insect’s body, killing it in the process. Free at last, it swims off to find a mate and continue the cycle.

Insect down. Parasite succeeds. Trout feast.

Why This Matters to Fly Fishing

For the fly fisher, understanding these natural rhythms and hidden ecological dramas can provide a serious edge. During late summer and early fall, terrestrials are an important part of a trout’s diet. Knowing that some of these bugs enter the water not by accident but by design helps explain the heightened feeding behavior seen on rivers and creeks during hopper season.

This also highlights why hopper patterns that land with a splash or flutter erratically can be so effective—they mimic not just the appearance of a land bug, but the unnatural, confused behavior of an insect under parasitic influence.

So the next time you're tying on a foam hopper or watching trout rise to a beetle pattern, remember: some of those insects weren’t blown in. They were pushed by something inside.

How to Fish the Hopper Bite: Matching the Mind-Controlled Meal

Fly fishing during hopper season is all about imitating chaos. These insects don’t land softly and drift peacefully like mayflies. They're twitchy, panicked, and often erratic—especially when they're under the influence of something like Spinochordodes tellinii.

1. Choose the Right Fly

Foam Hoppers: Patterns like the Chubby Chernobyl, Amy's Ant, and Dave's Hopper, create disturbance, and resemble a bug that’s out of place in the water.
Leggy Bugs: Flies with rubber legs and gangly profiles like Whitlock's Bright Spot Beetle imitate the flailing limbs of a struggling terrestrial.
Beetles and Crickets: Don’t overlook patterns like the Monster Beetle, Hegan's Ant, Dave Whitlock's Bright Spot Cricket patterns—especially around shaded banks.

2. Consider Fishing a Dropper

Hairworms aren’t just fantasy—they exist. Trout sometimes key in on worms after seeing them emerge from a dying insect. You can simulate this by tying a red or pink San Juan Worm or Squirmy Wormy as a dropper beneath your hopper. It’s not always “pure dry fly,” but it’s deadly effective.

3. Presentation Tips

Plop It Loud: These insects don’t land like ballerinas. Let your fly hit the water with a little drama—it signals a vulnerable meal.
Twitch It: Every few seconds, give the fly a slight twitch. Mimic an insect in distress or trying to self-rescue.
Target Edges and Undercuts: Terrestrials usually fall in from bankside vegetation, so focus casts near grassy edges, overhanging limbs, and riffle-to-pool transitions.
Midday Magic: Unlike traditional hatches, hopper fishing is best from late morning through the heat of the afternoon when insects are active and wind may knock them down.

Choosing the Right Rod for Hopper Season

Casting large, wind-resistant hopper patterns—or hopper-dropper rigs—requires more than just finesse. You need a rod that can turn over bulky flies, punch through wind, and still deliver with precision near tight banks. This is where the 6 weight is king.

Here are a few top-tier choices that excel during terrestrial season:

Orvis Helios 6wt
A powerhouse with laser accuracy. The Helios 3D is ideal for punching foam hoppers into headwinds and laying them down gently. Its backbone handles hopper-dropper rigs with ease, and the tip action still protects tippet.

Scott Centric 6wt
Fast, responsive, and extremely intuitive. The Centric gives you power when you need it but has a buttery flex that keeps casts smooth and roll casts easy. A perfect balance for hoppers in driftless creeks or big Western rivers.

Winston Pure 2 6wt
For anglers who prefer finesse and dry-fly feel, the Pure delivers a soft touch. Ideal for those fishing single hoppers in calmer conditions. The slower action allows for delicate presentations in tight quarters.

Scott Session 6wt
Session rods are high-performance hand-crafted fly rods that blend some of Scott's most acclaimed design approaches with their latest materials and technologies. This combination creates rods that bring together high line speed, exceptional loop control and accuracy with a light and lively feel in the hand.

Don't Forget the Floatants

For effective dry fly fishing, quality floatants like High N Dry, Loon, and Frog’s Fanny Double Duty are top choices. These products help keep your flies riding high and dry on the water. High N Dry offers both gel and liquid floatants with great all-around performance. Loon has a range including Aquel (gel), Fly Dip, and Dust (Shake n Bake powder). Frog’s Fanny Double Duty combines a desiccant and floatant in one, perfect for reviving soaked flies, especially CDC patterns.

Nature’s Darker Currents

Spinochordodes tellinii is just one example of parasites that alter host behavior to complete its life cycle. Others include Toxoplasma gondii, which makes rodents unafraid of cats, and Ophiocordyceps fungi, which turn ants into zombie spore spreaders.

These interactions remind us how complex, interwoven, and sometimes unsettling the natural world can be. As anglers, we're not just participating in a sport—we’re stepping into a web of ecological relationships older than we can imagine.

So enjoy Hopper Season. But know that beneath the splashy takes and twitchy strikes lies a story even more fascinating than the fish that eat the fly.