Parnassius smintheus is an alpine butterfly that overwinters as a first-instar caterpillar within its egg and often beneath the snow. While extreme temperatures in early winter appear pivotal to year-to-year population change of P. smintheus in the Rocky Mountains of Canada, the sources of mortality for these eggs are unclear. Here we tested three hypotheses about the vulnerability of eggs to warming and extreme weather in early winter (i.e. upper quartile of daily maximum and minimum microclimate temperatures in November): (1) warming disrupts the acquisition of cold tolerance, making eggs susceptible to subsequent cold snaps; (2) warming drives premature development and hatch, such that precocious hatchlings either starve or freeze; and (3) warming depletes the energy stores of dormant eggs. We then used these hypotheses to guide a simulation of the risk of winter mortality for eggs over the last half-century (1971 – 2020) in the Rocky Mountains of Canada. Early winter warming did not interrupt the acquisition of cold tolerance by freeze-avoidant eggs. Eggs did risk lethal freezing in simulated winters when extreme low temperatures coincided with an absence of snow cover. Early winter warming increased the risk of subsequent hatching, and precocious larvae were less cold-tolerant than eggs. Our simulation found that precocious larvae risked freezing during snow-free cold snaps in spring. Early winter warming did not appreciably drawdown energy stores, and we found that P. smintheus could not only fuel overwintering but tolerate several days of starvation after hatch. We conclude that eggs risk precocious development after early winter warming and are likely vulnerable in winters that lack persistent snow cover. Together, these sources of winter mortality may explain year-to-year change in P. smintheus populations. Identifying unidirectional thresholds, such as hatching and freezing, may be important for predicting the susceptibility of some alpine insects to future winter warming.
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