Pillbugs and sowbugs are really crustaceans that have come from the water but live totally on land. Pillbugs roll up when disturbed and sowbugs have two taillike appendages and cannot roll up.
Pillbug (left), Pillbugs on leaf (right)-University of Missouri-Columbia
They are grey, about 2cm long and have an armoured shell.
Sowbug (left), Sowbug (right)-J. kalisch
There are one or two generations per year. The female carries her eggs in a pouch on her undersides for up to two months. when the young hatch they stay in the pouch for a while longer before they emerge. In the winter they become inactive and hide in dark moist places. They occasionally are active in places like greenhouses or your basement in the winter. They also can live up to 5 years.
Diet and Damage
You can find them feeding in abundance near or in your compost heap, around organic materials, rotted plants and fruit and generally moist rich organic soil. They are rarely a problem in your garden since their diet consists mostly of organic material, but they can cause major problems on your young seedlings and tender young roots. They will eat plants and roots and do most of this damage at night. They can also invade a moist dark basement. I know in the Pacific Northwest where climate is moist and cooler there is an overabundance of these guys and they can be pests. When I lived in the Pacific Northwest they were a nuisance in my strawberry patch as well as most of my ornamentals. I would gather them in pails and walk over to the graveyard and dump them there. the only place you really want these guys is in your compost if at all.
1) Hand Pick
Collect them at night or when you see them in large masses and set them free somewhere else like the woods. If you want to kill them drop them in soapy water. You can even sweep them into a dust pan and dump them over the fence or somewhere else in your yard where there is ample organic material.
2) Make a Trap
Put rotted fruit under the plants they have been attacking and in the morning pick up the fruit with them on it and toss them far away.
3) Moisture Control
Try to minimize water usage and keep moist areas dryer. These guys tend to congregate in moist conditions.