What are bag worms? Bag worms are caterpillar-like worms that feed on your trees and can damage  your trees once they are established. For more information on bag worms visit https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/insects/caterpillars/bagworms.