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Drilled Sunflower Seeds
It seems the best bait for voles is to tie sunflower seeds to the trap trigger, but poking a needle through the seeds tends to split them.
Well, I can fix that:

Sunflower seeds – drilled This is “shell drilling” of a kind I had not previously encountered:

Sunflower seeds in shell – drilled Those are #52 drill holes, 63-ish mils in diameter, and pass a standard sewing needle with ease.
The traps have been baited and deployed amid a plague of voles and I await customer feedback …