P. napi is, generally, bi- or-multivoltine in the southern parts of its distribution and monovoltine in the north. Ten female butterflies were collected from the area surrounding Stockholm University on 12th June 2009 and placed in a rearing cage (0.5 × 0.7 × 0.7 m), in the laboratory, for egg laying. The adults were allowed to lay eggs for three consecutive days. The larvae hatching from the eggs were reared on garlic mustard, Alliaria petiolata, until pupation under a diapause regime, 20 °C; 12:12 LD (light-to-dark ratio) in 1.5 L plastic jars with five larvae in each jar. Pupae were stored at 1 °C for 8 months. On 30th March 2010, the overwintering pupae were transferred to 22 °C; 22:2 LD until adult eclosion. Five mating pairs of male and female butterflies from the overwintering generation were set-up in a rearing cage to produce offspring.