axlrosen points to
Boing Boing which points to
this horrible image gallery of caterpillar infestation in Sweden.
I'm so very sorry to present you with the images that will fill your nights with terror for the rest of the week. Entire bicycles and trees are coated with the cocoons.
The trees are stripped bare by this stuff, but apparently they come back.
A Boing Boing correspondent says this is probably the "Bird-cherry Ermine butterfly" but
Wikipedia suggests it's more of a moth.
What it'll look like when they're done. Not much improvement!