Wood lice feed at night on roots, stems, and leaves, and they live in damp shady areas, so I would suggest you move the hydrangea somewhere with more light, and less damp, you can get woodlice treatments from any garden centre. derekm

I don't wish to contradict Derekm here- but I was under the impression that woodlice played a sort of scavenger role by "mopping up" dead plant material and not eat live tissue??

They do indeed eat dead material, also live, if there are only a few I would leave them alone, but as unluckygardener says his plant is covered in them, I would take measures to get rid of them. derekm

Woodlice do eat very young seedlings but mainly eat rotting organic matter. They thrive in damp conditions. On balance they do more good than harm - much more. If they are a pest then some boiling water over them kills them.

But it doesn't do a lot for the hydrangea...

You won't ever see a Hydrangea covered in wood lice. As mentioned, they are ground insects that work and crawl at night. I would suspect that it is some other bug, such as aphids, mealybugs, or scale.

So can you post a photo?

I have a young hydrangea with budding stems about 40cm tall. A couple of days ago I noticed one stem had collapsed about 15cm from the top. The stem had been eaten away on one side causing the collapse. Today another stem has suffered the same fate. I went out after dusk and found two creatures that looked like tiny woodlice. One was on a stem which was starting to show similar damage, and I notice that other stems are being similarly eaten away although not yet right through to the hollow centere. Are these woodlice, or some other creature?