I presumed these were aphids covering a branch of a native beech alongside a mountain trail, but I’ve never seen anything like it. Quick research tells that they are beech blight aphids, known commonly as the boogie woogie aphid. They are typically still while feeding, but they sway frantically when approached. I am easily entertained, but this lightened the load of the uphill hike.
I am very forgiving of insects in my garden, but I’m not certain I’d be able to overlook this invasion, even if the aphids will do no more than kill a single branch of the mature tree.