Rainbow Caterpillars once infested the Great Ga'Hoole Tree during the Age of Enlightenment. They spun beautiful multicolored globules of silk shaped like milkberries. They were named luminaspheres, and the owls of the tree would sit for hours just to observe their beauty. The caterpillars were forbidden to be eaten, but this proved catastrophic as the caterpillars began to eat the leaves and shoots of the Great Tree, stunting its growth.
It was only when Heilo the Blind sensed the source of the tree's sickness that the caterpillars were ordered to be killed off, and the Guardians learned to be wary of threats to the Great Tree.