AbstractThere was no difference in dose response to abamectin between a laboratory strain of Colorado potato beetle(Leptinotarsa decemlineata (Say))from North Carolina and a multiple resistant strain from Massachusetts(LD50= 1.95 and 1.98 ng larva−1, respectively). Two abamectin‐resistant strains were generated by separate means. The mutagen, ethyl methanesulfonate, was used to generate an abamectin‐resistant strain (LD50= 29.4 ng larva−1) from the susceptible laboratory strain. The other resistant strain(LD50= 45.3ng larva−1) was generated through an intense selection with abamectin of a field strain contained in cages set up in existing potato fields. Resistance ratios calculated fromLD50values for the two abamectin‐resistant strains were 15‐fold and 23‐fold, respectively. Resistance ratios calculated fromLD97values(21‐fold and 38‐fold, respectively) were higher than those calculated atLD50. Also, there were larvae in both resistant strains which were capable of surviving doses up to 100 ng larva−1, while the susceptible strain had no survivors at 10 ng larva−1. Although the logit mortality regression analysis produced statistically different lines for the two abamectin‐resistant strains, the slopes of each appear to be the same. Both resistance factors were determined to be autosomal and incompletely recessive (0.26 to 0.4, respectively) via reciprocal F1crosses to the susc