Abstract: The developmental rate of codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), is supposed to be directly proportional to air temperature between the lower and upper developmental thresholds. However, some review papers suggest that insect species have a higher developmental rate when reared in thermoperiodic conditions as compared with constant temperatures. Thus, in this study, the developmental rate and the zero temperature threshold of codling moth for the larval stage were determined in thermoperiodic conditions for strains resistant and non-resistant to chemical and viral (CpGV) insecticides. Two methods were used to determine the zero development temperature for the four C. pomonella strains: (i) the x-intercept method and (ii) the thermal unit test. Our study supports the “thermoperiod hypothesis” and suggests that the effect of thermoperiod on the developmental rate of C. pomonella larvae should be taken into account in the development of phenological models.