Green Rock Worm
Rhyacophila spp.
- Hook size
- #12-16
- On the water
- Apr 15 – Oct 15
- Peak
- May 20 – Jul 20
- Time of day
- All day
Every other caddis larva in this guide builds something — a case, a net, a retreat. The Green Rock Worm builds nothing at all: it hunts out in the open on bare rock, and that's exactly what makes it one of the most reliable nymphs of the whole year.
Telling it apart
- Bright green to olive larva, with no case or shelter of any kind — bare on the rock.
- A robust, arched body, with well-developed legs for actively moving rather than staying put.
- Present year-round, in almost every type of current — not tied to one specific hatch.
- A predatory larva, unlike most other caddisflies, which filter-feed or graze.
Easily confused with
A dislodged Little Black Caddis larva, but that one is much smaller and never as green. Don't confuse it with a case-building caddis larva — the Green Rock Worm never builds one at all.
A hunter, not a filter-feeder
Unlike the Spotted Sedge, which spins a net, or the Grannom, which carries a case, the genus Rhyacophila's larva builds nothing at all. It moves freely over the rocks and actively hunts other small invertebrates — predatory behaviour that's rare among caddisflies, making it almost a small aquatic equivalent of a hunting caterpillar.
That total lack of shelter leaves it especially exposed to current: the smallest surge of water dislodges it and sweeps it into the drift, far more often than a larva protected inside a case or a net.
Always in the drift, never tied to one hatch
Because it has no fixed retreat, this larva regularly ends up swept into the current year-round, with no tie to a specific synchronized emergence. That makes it a reliable nymph twelve months a year rather than a pattern you pull out for a two-week window.
It turns up in almost every kind of current — rapids, riffles, moderate seams — which makes it a versatile choice when nothing specific seems to be hatching.
One of the best nymphs between hatches
When nothing is visibly hatching and trout are still feeding below the surface, a green Rock Worm–style nymph covers a good share of what's actually drifting — since this larva ends up in the current continuously rather than in response to one specific trigger.
Fish it near bottom, in the same steady-current seams it lives in, on a natural drift with no particular animation.
Fishing it, stage by stage
Larva
Year-round, in nearly every kind of current.Near bottom, on a natural drift, with no animation.
Every other insect worth knowing, month by month: See the full hatch chart