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J. Agric. Urban Entomol. 27: 61-73 (2010/2011) ABSTRACT The Colorado potato beetle Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae) in the northern part of the Xinjiang Uygur Autonomous Region has evolved resistance to several pyrethroid and carbamate insecticides. Biological control methods should be a major component of integrated pest management for L. decemlineata. Spinosad and abamectin are two biopesticides that have unique mechanisms of action. In this study, the contact toxicities of spinosad and abamectin to L. decemlineata fourth instars and adults were determined by topical applica- tions to several field populations. The average LD50 values of spinosad and abamectin for adults were 0.1275 and 0.0101 mg (a.i.) per individual, and for fourth instars they were 0.0181 and 0.0016 mg (a.i.) per individual, respectively. These data were among the lowest LD50s ever estimated, which affirmed that the two biopesticides are useful for L. decemlineata control in north Xinjiang. Susceptibilities to the two biopesticides varied slightly but significantly among tested field populations and the variations did not result from cross-resistance to conventional insecticides. Regarding stomach toxici- ties, the LC50 values of spinosad applied to excised potato leaves for second instars, third instars, fourth instars, and adults were 0.2840, 0.4093, 1.2413, and 2.3783 mg/L (a.i.), respectively. The LC50 values of abamectin for second instars, third instars, fourth instars, and adults were 0.0036, 0.0088, 0.0177, and 0.2591 mg/L (a.i.), respectively. The two biopesticides were most toxic to second instars, followed by third, then fourth instars, and they were least toxic to adults. These data suggested that the appropriate timing for spinosad or abamectin spraying is to early larval stages.