Disclosed is a method for preventing damage to trees caused by harmful insects by injecting into a tree trunk a composition containing a neonicotinoid-based insecticide selected from clothianidin and dinotefuran, a solvent miscible with water and a surfactant, wherein the surfactant contains at least one type selected from the group consisting of polyoxyethylene hardened caster oils and polyoxyethylene styryl phenyl ethers.