What's eating my rhubarb!

My rhubarb crowns are being munched to death by something, they are covered up with a plastic cloche but some beastie is still getting to them.  Will it be underground slugs? (can't remember the correct name for them).  I am planning to get some nemaslug, just hope it is not too late for the rhubarb.    The crowns have only just been planted this year so maybe they are more vulnerable - I thought rhubarb leaves were poisonous anyway.

I would remove the plastic and sprinkle some pellets around the area,but saying that ive never had slugs eating mine