probably walnut caterpillars (Datana integerrima)

They look like the ones that were on my Q. Robur Eng oak last year about now, late summer. I thought tent cattapillar. I didn't treat last late summer, because the tree would be going dormant soon in fall anyway. But when tent cattapillars swarmed this spring, I put Sevin dust on my tree, on the leaves, and onto the "TENTS". I will try BT "Bacillus Thuringis" (May be misspelled) on the foliage. Sevin kills "Good" bugs as well as bad, so next spring, I will try to use the BT that affects worms only. Too bad some worms are butterflies eventually. Maybe the butterfly worms come along after the bagworms are gone? Who knows. It is hard to only affect the bad things, without affecting the good bugs.