The first study of pesticide resistance in populations of citricola scale was initiated in 2006 in the San Joaquin Valley. A bioassay method was developed using excised leaves dipped in chlorpyrifos. Scales increased their resistance to chlorpyrifos as they grew in size over a 3 month period. There were significant differences in the mean response of scales to discriminating concentrations of chlorpyrifos signaling the presence of resistance.