Effects of wood vinegar on the activity of various insecticides were determined by measuring the mortality of two species of rice planthoppers, Nilaparvata lugens and Laodelphax striatellus. Wood vinegar itself did not show insecticidal activity on planthoppers. When the planthoppers were treated with wood vinegar mixed with one of insecticides such as BPMC, dinotefuran, imidacloprid, carbosulfan or insect growth regulators, the planthopper mortality induced by carbosulfan was greatly increased by the wood vinegar in comparison with a single carbosulfan treatment. Wood vinegar showed no effect on other insecticides. In addition, the wood vinegar-carbosulfan mixture significantly reduced AChE activity of planthoppers, which is a target molecule of carbosulfan. This result suggests that wood vinegar has a synergistic effect on the insecticidal activity of carbosulfan. Our study provides information on a potential role of wood vinegar in facilitation of activity of specific insecticides.