Encourage natural predators like birds, hedgehogs, and ground beetles into the garden. Hedgehogs like dense undergrowth and grasses for hiding and nesting. You can also build or purchase hedgehog homes.
A mix of pathogenic nematodes can be purchased from biological control suppliers. These microscopic organisms enter caterpillar bodies and infect them with a bacterial disease. Spray nematodes in September and October on trunks, branches, and the soil under infrected trees.