The bioactivity of the chitin synthesis inhibitors, chlorfluazuron, hexa‐flumuron, teflubenzuron, triflumuron and GR 572(novaluron) used in commercial or experimental formulations against larvae of Colorado beetle, Leplinotarsa decemlineata Say in laboratory and field experiments was evaluated during 1985–90. All compounds showed high potential for control of the pest. The activity mainly depended on the period of time after treatment. Higher activity was found on younger larvae. A shailow dose‐response relationship was observed for a given larval stage, particularly at the last observation, 15 days after treatment. A single treatment is able to suppress the pest population density below a dangerous level during the whole season. Observations showed that long‐term activity resulted from stomach and contact larvicidal action, reduced egg viability and antifeedant effect.