As the growing season is expected to begin earlier under climate change, insects should initiate reproduction several days or weeks earlier than they used to. In eastern Canada, hemlock looper (HL) Lambdina fiscellaria (Guenée) (Lepidoptera: Geometridae) females generally oviposit in September, with eggs entering an obligatory diapause quickly after their deposition. We therefore simulated an early start of the HL reproduction cycle of 2, 4, 6, or 8 weeks to examine the extent to which freshly laid eggs from two populations (island and mainland) can withstand exposure to four temperature conditions (15, 20, 25 °C, or fluctuating temperature in an outdoor insectary), with all treatments ending on 1 September 2007. On this date, half the eggs from each population were immediately incubated at 15 °C, while the rest were stored in an outdoor insectary until their incubation at 15 °C the following spring. In a separate experiment, the effect of temperature on pre-diapause duration was determined from the number of days required for eggs to change colour after oviposition. The pre-diapause phase was completed faster as temperature increased. Regardless of incubation date and population, percent hatch decreased significantly after 6-8 weeks of exposure to 25 °C or in the outdoor insectary. Under most treatments, the odds of dying as pharate larvae increased with exposure duration. When eggs were incubated at 15 °C immediately after treatment, time to hatch and diapause duration remained constant over treatments, except at 25 °C when they both decreased. After 8 weeks of exposure to 15 or 20 °C, eggs transferred outdoors were more likely to hatch precociously than those exposed to 25 °C or insectary conditions. Globally, mortality seemed greater among eggs stored outdoors than among those kept indoors. Most eggs that survived the winter hatched synchronously after incubation in spring. Overall, larger eggs from the island population survived better than smaller eggs from the mainland population.