This Swedish website contains jaw-dropping (and way grody) photos of a carpet caterpillar (?) infestation in which bicycles and trees are completely covered in cocoon that’s filled with masses of writhing, maggot-like larvae.
(Thanks, Fipi Lele!)
Update: Thomas sez, “The larvae are almost certainly from the butterfly Yponomeuta evonymella, Bird-cherry Ermine (or Häggspinnmal in Swedish). It has haunted areas of Sweden for several years, but the trees do come back after being stripped clean of their foliage.”