The spotted tentiform leafminer, Phyllonorycter blancardella (F.), overwinters in a photoperiodically induced state of pupal diapause. The critical day length (50% of leafminers enter diapause) for P. blancardella from latitudes of about 42–44°N in southwestern Ontario was 14.25 h at an average August temperature (Environment Canada 1993) of 20°C (Trimble 1994). The development time of actively growing larvae decreased with increased temperature, whereas the development time of diapause-destined larvae was not affected by temperature at the critical day length, suggesting a strong interaction between photoperiod and temperature in the regulation of prediapause development (Trimble 1994). This paper describes the interaction of photoperiod and temperature in the regulation of the rate of egg and larval development of diapausedestined leafminers in the laboratory and in semi-natural conditions.