First-, third-, and fifth-instar Epiphyas postvittana (Walker) were exposed to a range of lufenuron concentrations (0–200 ppm) incorporated into synthetic diet and their subsequent development and mortality responses were determined. For all instars the greatest change in mortality response occurred over lufenuron concentrations ≤3 ppm. However, third and fifth instars displayed an increase in mortality earlier than first instars, and were more sensitive to the lower lufenuron concentrations in this range. Only first and third instars subjected to ≤2.5 ppm lufenuron survived the 26-d exposure trial. No larvae first exposed to lufenuron as first or third instars survived to pupation if ingesting concentrations of ≥1 and ≥3 ppm, respectively. Consumption of lower lufenuron concentrations by these larvae delayed pupation and resulted in pupal deformity. In contrast, fifth instars subjected to 100 ppm were capable of surviving the 26-d trial period and displayed a slower progressive reduction in survival to pupation with increase in lufenuron concentration. Also in contrast to more immature stages, fifth instars exposed to lufenuron developed more rapidly to pupation than larvae not exposed to the insect growth regulator (IGR), and all resulting pupae were normal. Third instars were exposed to sublethal lufenuron concentrations (0–3 ppm) for 4 d and the fourth-instar survivors subjected to a controlled atmosphere cold storage treatment (2% O₂, 2% CO₂, 0.6°C). Larvae ingesting diet containing 0.5 ppm (and to a lesser extent 1 ppm) lufenuron required longer exposure to the postharvest treatment to achieve ≥95% mortality than larvae not ingesting the IGR. However, the analogous mortality response of larvae exposed to 3 ppm lufenuron was comparable to the control.