Grannom
Brachycentrus spp.
- Hook size
- #14-16
- On the water
- Apr 25 – Jun 5
- Peak
- May 5 – May 25
- Time of day
- Morning
This is the signal that spring has really arrived: the year's first big caddis, often on the very same day as the Hendrickson. The black swarms over the riffles are hard to miss, but the real action happens twice — once in the morning at emergence, once in the afternoon when the females come back to lay.
Telling it apart
- A very dark brown to black adult, size 14–16, wings held tent-like at rest like all caddisflies.
- One of the season's first big caddis hatches, often right alongside the Hendrickson in late April–early May.
- The larva builds a four-sided, tapering case cemented from plant fragments — that square cross-section is the fastest tell, quite different from the round, pebble-built cases of many other case-making caddis.
- Dense, fluttering swarms over riffles and streamside vegetation at the peak of the hatch.
Easily confused with
The Little Black Caddis (Chimarra aterrima), on the water at the same time over the same riffles, but roughly half the size (18–20) — easy to mistake for a midge. The Grannom, by comparison, is clearly bigger and easy to pick out with the naked eye.
The square case
The Grannom larva moves around sealed inside a tapering, square-cross-section case built from small plant fragments cemented with silk — a detail that sets it apart at a glance from round-cased caddis or species that live free without any shelter. It uses the case while grazing and filtering fine organic matter in moderate, well-oxygenated current.
You typically find it clinging to submerged vegetation and woody debris rather than bare rock — that is where it finds both its building material and its food.
Spring's first big dries
Coming this early in the season has an advantage: few other large insects are competing for the attention of trout coming out of a lean winter. Unlike the March Brown's steady trickle, the Grannom often emerges in a genuine synchronized wave, especially on mild, overcast mornings — producing dense, unmistakable flights.
On a river where Hendrickson and Grannom overlap, the two hatches can blur into one rise of fish taking either indifferently — one of the few times all year the exact pattern matters a little less than usual.
The afternoon return
Once mated, the females do not just tap the surface to lay — they come back in the afternoon and actually dive or crawl underwater to glue their eggs onto a submerged surface. That behaviour opens a second fishing window, distinct from the morning emergence.
At that point trout are watching for the insect swimming actively underwater just as much as the one floating on top. A fly given a little movement — small twitches or strips — imitates that far better than a perfectly dead drift.
Fishing it, stage by stage
Pupa / emerger
Morning, at emergence.A pupa fished on a rising drift toward the surface, or just under the film right at emergence.
Adult
Dry in the morning, for egg-laying in the afternoon.A dark dry dead-drifted in the morning. In the afternoon, a wet fly worked with small strips imitates the diving, egg-laying female — any greenish nymph fished with real movement does the job; it does not need to be an exact imitation.
Every other insect worth knowing, month by month: See the full hatch chart