Development of resistance to organophosphorus and carbamate insecticides by the Oriental fruit moth, Grapholita molesta (Busck), has caused great concern regarding the sustainability of peach production in Ontario, Canada. Thus, a resistance management program was implemented and a procedure was developed to monitor for resistance to insecticides in adult moths. Diagnostic concentrations needed to distinguish susceptible genotypes from resistant individuals were determined to be carbofuran (0.1 μg per vial), carbaryl (1.0 μg per vial), malathion (0.5 μg per vial), and cypermethrin (2.5 μg per vial). Significant survival of field-collected moths to diagnostic concentrations of cypermethrin suggested that resistance to pyrethroids might be widespread. The seasonal patterns of resistance of several Oriental fruit moth populations from the Niagara Peninsula of Ontario indicated a decrease in frequencies of resistance to all insecticides tested from 1996 through 1998. Resistance levels were unstable, thus suggesting that resistance could be successfully managed. Our data confirmed the usefulness of the vial test to monitor for resistance to insecticides in the adult Oriental fruit moth, and provided the basis for the development of a resistance management strategy designed to extend the efficacy of all classes of insecticides used for control.