 Tillyard did good work in New Zealand and established a reputation for his work on the biological control of plant and insect pests.He is popularly best known for his introduction of a small wasp as an agent for controlling woolly aphis in apple-trees.In 1925 he was elected a fellow of the Royal Society, London, and in the following year he published his book on "The Insects of Australia and New Zealand", a comprehensive work with many illustrations.