Some insect species are thought to grow faster even under low temperature in natural conditions, presumably utilizing infrared rays from sunlight by basking behavior. Parnassius citrinarius is one of these species. The larvae of this butterfly in the field, despite of low ambient temperature of early spring, grow much faster than those in the laboratory. They attain this enhanced growth by choosing warmer place to raise their body temperature. But the process how this short larval period increases their fitness still remains unexplored.

One of the most plausible hypotheses is the avoidance of natural enemy. The longer the larval period is, the higher the risk of predation becomes. The larvae are suffered by parasitoid fly even in such cold season. So we surveyed seasonal change in the density of parasitoids using yellow sticky trap.

Another reasonable hypothesis is synchronization with the phenology of food plant. Their major food plant, Corydalis incisa, emerges in late autumn, passes the winter as a rosette form, starts to grow at early spring, and then withers when summer comes. Hence, prolonged larval period may force them to feed on well grown hard leaves. We estimated the effects of food plant (young or old) on larval performance measuring their larval period and pupal weight.