Tarpaper for slugs?

A friend recommended I use tarpaper collars around my plants to ward off banana slugs (rapicious little buggers here in N. California).
Any comments on the effectiveness of this? Anyone tried it?
I know about beer traps by the way. And I will not use metaldehyde.
cheers
Lawrence
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I've been using the iron phosphate slug killer this year, and so far so good. It looks a lot better than tarpaper.
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