This trap is full of beer and is then dug in so the lip is level with the soil. The slugs can slither easily into the bowl and for some reason seem to be attracted to the smell of the beer. When they go in, they get pickled and die.
The lid that fits on top stops rain and irrigation filling the bowl and diluting the beer. Its simple but effective. It seems to have attracted alot of slaters also which is a shame.