Seasons impose different selection pressures on organisms through contrasting environmental conditions. How such seasonal evolutionary conflict is resolved in organisms whose lives span across seasons remains underexplored. Through field experiments, laboratory work, and citizen science data analyses, we investigate this question using two closely related butterflies (Pieris rapaeandP. napi). Superficially, the two butterflies appear highly ecologically similar. Yet, the citizen science data reveal that their fitness is partitioned differently across seasons.Pieris rapaehave higher population growth during the summer season but lower overwintering success than doP. napi. We show that these differences correspond to the physiology and behavior of the butterflies.Pieris rapaeoutperformP. napiat high temperatures in several growth season traits, reflected in microclimate choice by ovipositing wild females. Instead,P. rapaehave higher winter mortality than doP. napi. We conclude that the difference in population dynamics between the two butterflies is driven by seasonal specialization, manifested as strategies that maximize gains during growth seasons and minimize harm during adverse seasons, respectively.