Looks like what I'd call a woodlouse. There are hundreds of local names for them - slaters, pea bugs, pill bugs, roly polies, chucky pigs and there are many different species. Whether they do any harm is a moot point. One school of thought maintains they eat only dead and decaying plant material. Others maintain stoutly that they have seen them with their own eyes eating young plants.

Here is a link that might be useful: Pill bug threads

Thanks floral