α-Mangostin, a major xanthone derivative in mangosteen (Garcinia mangostana), has been proven to have a wide range of biological-activities. In this study, we assessed the larvicidal and antifeeding activities of α-mangostin using larvae and adults of the Colorado potato beetle, Leptinotarsa decemlineata Say. α-Mangostin showed larvicidal activity and growth inhibitory effects under non-choice feeding conditions. The LC₅₀ values with 7-, 14- and 23-day treatment were 63.66 (13.29–279.79) μM, 8.37 (2.67–20.27) μM and 4.09 (0.89–11.81) μM, respectively. Larval development was significantly delayed 3–8 days with α-mangostin treatment. The antifeeding activity of α-mangostin was also tested by leaf disc non-choice and choice methods against larvae and adults of L. decemlineata. α-Mangostin had stronger antifeeding-activity toward adults than toward larvae, whereas there was low lethality in adults associated with higher antifeeding indexes (AFIs). The larvicidal and antifeeding effects suggest that α-mangostin and related compounds should be explored for their potential as botanical insecticide alternatives for control of the Colorado potato beetle.