Abstract The susceptibility of tufted apple budmoth larvae, Platynota idaeusalis, to azinphosmethyl decreases with each successive instar. A comparison between fifth and third instars showed that the fifth instars have a higher level of glutathione S-transferase activity per milligram of protein, a lower content of cytochrome P-450 per milligram of protein, and absorb a lesser percentage of and LD 01 dose than the third instar. Results of metabolism were consistent with these findings. In addition to these factors, the fifth instar larvae may have a threshold mechanism for eliminating penetrated azinphosmethyl from the body, unchanged. This allows the fifth instar larval population to withstand increasingly higher doses of azinphosmethyl without a proportional increase in mortality.