Here in KY, we had a problem a few years ago with foals mysteriously dying. In horse country, this was very upsetting! They finally realized that it was the tent worms that were killing them. The tent worms that had nested (and feasted) in cherry trees would fall to the ground and be eaten by the foals as they were grazing in the grass. Since the foals were so much smaller than their mothers, there were enough toxins ingested by the worms from the cherry trees to cause the foals to sicken and die. So they can cause other problems even more serious than the damage they do to trees.
Donna