External conditions can drive biological rates in ectotherms by directly influencing body temperatures. While estimating the temperature dependence of performance traits such as growth and development rate is feasible under controlled laboratory settings, predictions in nature are difficult. One major challenge lies in translating performance under constant conditions to fluctuating environments. Using the butterfly Pieris napi as model system, we show that development rate, an important fitness trait, can be accurately predicted in the field using models parameterized under constant laboratory temperatures. Additionally, using a factorial design, we show that accurate predictions can be made across microhabitats but critically hinge on adequate consideration of non‐linearity in reaction norms, spatial heterogeneity in microclimate and temporal variation in temperature. Our empirical results are also supported by a comparison of published and simulated data. Conclusively, our combined results suggest that, discounting direct effects of temperature, insect development rates are generally unaffected by thermal fluctuations.

The life cycles of animals vary in relation to local climate, as a result of both direct environmental effects and population‐level variation in plastic responses. Insects often respond to the approach of winter by entering diapause, a hormonally programmed resting state where development is suspended and metabolism suppressed. Populations often differ in the duration of diapause, but the adaptive reasons for this are unclear. We performed a common‐garden overwintering experiment with respirometric measurements in order to investigate the progression of diapause in the butterfly Pararge aegeria. Both the duration of diapause and the depth of metabolic suppression were shown to vary between populations. In contrast to previous results from various insects, diapause duration did not correspond to the local length of winter. Instead, the observed pattern was consistent with a scenario in which diapause duration is primarily a product of selection for suppressed metabolism during warm autumn conditions. The relationship between optimal diapause duration and the length of the warm season is complicated by variation in the number of yearly generations (voltinism). These results shed new light on variation in diapause ecophysiology, and highlight voltinism as an integrated product of selection at multiple points in the seasonal cycle. A free Plain Language Summary can be found within the Supporting Information of this article.

Temperature influences the rate of most biological processes. Nonlinearities in the thermal reaction norms of such processes complicate intuitive predictions of how ectothermic organisms respond to naturally fluctuating temperatures, and by extension, to climate warming. Additionally, organisms developing close to the ground experience a highly variable microclimate landscape that often is poorly captured by coarse standard climate data. Using a butterfly population in central Sweden as a model, we quantified the consequences of small‐scale temperature variation on phenology, emergence synchrony and number of annual reproductive cycles (voltinism). By combining empirical microclimate and thermal performance data, we project development of individual green‐veined white butterflies (Pieris napi) across 110 sites in an exceptionally high‐resolved natural microclimate landscape. We demonstrate that differences among microclimates just meters apart can have large impacts on the rate of development and emergence synchrony of neighbouring butterflies. However, when considering the full development from egg to adult, these temporal differences were reduced in some scenarios, due to negative correlations in development times among life stages. The negative correlations were caused by temperatures at some sites beginning to exceed the optimum for development as the season progressed. Indeed, which sites were optimal for fast development could change across the lifetimes of individual butterflies, that is, ‘fast’ sites could become ‘slow’ sites. Thus, from a thermal point of view, there seem to be no consistently optimal microsites. Importantly, the fast sites were not always the warmest sites. We showed that such unintuitive effects could play an important role in the regulation of phenological synchrony and voltinism in insects, as most sites consistently favoured two generations. The results were generally robust across years and three different egg‐laying dates. Using high‐resolved empirical climate data on organism‐relevant temporal and spatial scales and considering nonlinear responses to temperature, we demonstrated the large and unintuitive population‐level consequences of locally and temporarily high temperatures. We suggest to—whenever possible—incorporate species‐ and life stage‐specific nonlinear responses to temperature when studying the effects of natural microclimate variation and climate change on organisms. Read the free Plain Language Summary for this article on the Journal blog.

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 rapae and P. napi). Superficially, the two butterflies appear highly ecologically similar. Yet, the citizen science data reveal that their fitness is partitioned differently across seasons. Pieris rapae have higher population growth during the summer season but lower overwintering success than do P. napi. We show that these differences correspond to the physiology and behavior of the butterflies. Pieris rapae outperform P. napi at high temperatures in several growth season traits, reflected in microclimate choice by ovipositing wild females. Instead, P. rapae have higher winter mortality than do P. 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.