It was entirely covered in webbing, revealing the culprit to be the euonymus caterpillar, larva of the spindle ermine moth. This is a specialist in the spindle bush, a European euonymus, and came over here with that shrub. Once it got to North America, it found that there was another shrub already here that tastes just as good.
Last year, as I wrote here, this wahoo was badly infested, but not stripped. But Yponomeuta cagnagella is so specialized that it not only feeds on that special euonymus, it also cocoons there, and then apparently returns to lay its eggs there. We have three of these around the property--this one in the front yard, one back further in the growing young forest, and one in a far corner, nestled into a growing lilac hedge (it wasn't planted that way, but the lilacs are doing very well). The more central tree had a few of these caterpillars on it (easily removed), the one in the far corner was completely covered by them, but these had hatched a little later (further north, don't you know). That tree is only half eaten, and I spent a good hour each day yesterday and the day before removing caterpillars. The caterpillars spend a great deal of time huddled in masses, inside their webs (particularly in the morning when it's still pretty cool), so are pretty easy to pick off the branches in handfuls. Saturday, I was tossing them onto the road, yesterday I had a nice pail of soapy water for them. Yesterday I picked more than a litre of the critters. Some remain because they were out of reach--I'll be out again in the hopes that they will collect lower down.
The soapy water method (dish soap) works very well--and when you can toss them into a bucket, instead of washing down the tree there's no risk to the tree--the soap will burn the leaves of some species if left on too long.
The stripped tree is the most interesting case--infestations such as it had are good for no one. I found no cocoons on it, and lots of clumped together caterpillars, not yet full grown, slowly starving, having eaten their entire food supply. Lots of them had also moved to the adjacent Norway maple, to huddle and starve--they can't (won't) eat maple leaves. These guys get around on silk ropes, and apprently can't get back to the tree if these are broken, nor it seems can they travel any distance to find a new host--perhaps have no way of discerning one. So I think the danger to this particular tree is over, if it survives. I am concerned that the sticky webbing in which it is covered will hamper its efforts to leaf out again, which will reduce its chances for survival. During the soapy water attack on the caterpillars on the other tree, I noticed that the stickiness of the webbing is completely defeated by the detergent. Webbing and caterpillars stuck to my hand dropped right off when immersed in the soapy water. So I am considering washing down the stripped tree with soapy water in the hopes that at least some of the webbing can then be rinsed off with a blast of the hose.
I tried a variety of techniques for vanquishing these caterpillars, including exposing them on rocks, and as mentioned, throwing them on the road, to see if anyone would like to eat them. I did see an ant bite one of the ones on the rock, but don't know if it ended up killing and taking it. And I watched the spider above grab hold of one a few times, and apparently decide that it wasn't for her. Another condition necessary for an infestation such as this one is that there are few or no predators, which may well be the case here.