It sure can be frustrating when they're on the willow grub. I remember countless hours watching large Hautapu browns sitting under overhanging branches gorging themselves on the little buggers and completely ignoring anything else.
I use to tie lemon coloured thread on a #14 0r #16 caddis hook and fish it on a 15-20cm dropper under a Blowfly Humpy with reasonable results.
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