Blast them off the leaves with a hose, then tell all the magpies, flickers, crows, chickens, badgers, skunks or whoever to come get 'em before they climb up the tree again. Water blasting is the best way to get most localized pests off your plants and it won't hurt the tree.
I suck at caterpillars, so I can't tell you what species they are. They don't look like tent caterpillars, anyway. Many caterpillars with hairs have a bit of a sting, so be careful if you handle them.
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