When is the Brown Beetle most available to trout?

October–December, typically 19:00–23:00 (dusk into dark). It picks up from about 12°C, peaks around 17°C and tails off by 23°C.

What flies imitate the Brown Beetle?

Brown Beetle, Yellow Humpy, Foam Beetle, typically on a #10–8 hook.

How big is the Brown Beetle?

Adult stage: 12–16mm. That matches a #10–8 hook.

Terrestrial · Terrestrial

Brown Beetle

Costelytra giveni

Size

12–16mm

Hook size

#10–8

Hatch window

Season October–DecemberPeak time 19:00–23:00 (dusk into dark)

Water temperature

17° peak

12°23°

How fish feed on it

Keep this one for last light, not the rest of the day — it has no business in a daytime box. Grass grub beetles fly at dusk and on into full dark on warm evenings from October through December, once the air itself is holding real heat, and it's around lake margins and lowland water specifically that the fall happens, as beetles lift off the surrounding pasture and blunder out over the surface as the light goes. Most evenings there's nothing to show for it; then one warm dusk the margins are full of working fish and it's the only fly worth having on. A single beetle pattern earns its place in an evening box for exactly this window and does nothing for you at any other time of day.

Flies that imitate this

Brown Beetle

Yellow Humpy

Foam Beetle

Ecology

Rivers & stillwaterLowland lochsPeaty loughs

Where you'll find this

New ZealandPresent

Present throughout

Identification

Costelytra giveni (mischaracterized as Costelytra zealandica prior to 2016), commonly known as New Zealand grass grub , is a scarab beetle that is endemic to New Zealand and is a prevalent pasture pest.