Effects of constant temperatures and thermoperiods on larval and pupal development and survival were investigated in the tufted apple bud moth,Platynota idaeusalis(Walker). Development and survival were either the same, decreased, or increased under thermoperiods as compared to mean temperatures depending upon the temperature regime tested. Development and survival at 12-h fluctuating temperatures between 20 and 33°C were the same as mean constant temperatures of 24 and 27°C. Fluctuating temperatures with thermophases>33°C decreased development time and survival as compared to thermophases ≤33°C. Cool fluctuating temperatures increased development rate and survival relative to mean constant low temperatures of 13 and 18°C and they also permitted development and survival to occur at thermal levels below the lower development threshold as calculated by constant temperatures.