Ecotones linking open and forested habitats contain multiple microhabitats with varying vegetal structures and microclimatic regimes. Ecotones host many insect species whose development is intimately linked to the microclimatic conditions where they grow (e.g., the leaves of their host plants and the surrounding air). Yet microclimatic heterogeneity at these fine scales and its effects on insects remain poorly quantified for most species. Here we studied how interspecific differences in the use of microhabitats across ecotones lead to contrasting thermal exposure and survival costs between two closely‐related butterflies (Pieris napi and P. rapae). We first assessed whether butterflies selected different microhabitats to oviposit and quantified the thermal conditions at the microhabitat and foliar scales. We also assessed concurrent changes in the quality and availability of host plants. Finally, we quantified larval time of death under different experimental temperatures (thermal death time [TDT] curves) to predict their thermal mortality considering both the intensity and the duration of the microclimatic heat challenges in the field. We identified six processes determining larval thermal exposure at fine scales associated with butterfly oviposition behavior, canopy shading, and heat and water fluxes at the soil and foliar levels. Leaves in open microhabitats could reach temperatures 3–10°C warmer than the surrounding air while more closed microhabitats presented more buffered and homogeneous temperatures. Interspecific differences in microhabitat use matched the TDT curves and the thermal mortality in the field. Open microhabitats posed acute heat challenges that were better withstood by the thermotolerant butterfly, P. rapae, where the species mainly laid their eggs. Despite being more thermosensitive, P. napi was predicted to present higher survivals than P. rapae due to the thermal buffering provided by their selected microhabitats. However, its offspring could be more vulnerable to host‐plant scarcity during summer drought periods. Overall, the different interaction of the butterflies with microclimatic and host‐plant variation emerging at fine scales and their different thermal sensitivity posed them contrasting heat and resource challenges. Our results contribute to setting a new framework that predicts insect vulnerability to climate change based on their thermal sensitivity and the intensity, duration, and accumulation of their heat exposure.

18Vegetation cover generates local microclimatic gradients in the understorey, being especially 19 pronounced at narrow ecotones linking open and forested habitats (open-closed ecotones). 20They provide key habitats for multiple insect communities and may largely determine the 21 exposure of herbivorous insects to the increasing impacts of climate change. We report parallel 22 measurements of microclimatic variables, multiple host-plant traits, and oviposition behaviour 23 in Mediterranean populations of two Pieris butterflies across ecotones of vegetation cover. 24Open microhabitats were significantly warmer, drier, and more exposed to thermal 25 amplification, which increased temperatures to values affecting insect larval survival. Host plants 26 advanced their reproductive phenology and were shorter. Open microhabitats also inhibited the 27 development of shade-adapted plants (e.g. Alliaria petiolata), decreasing fruit production. In 28 contrast, the reproduction of sun-adapted host plants (e.g. Lepidium draba) was vigorous in the 29 open microhabitats and completely inhibited in closed microhabitats, which were exclusively 30 inhabited by non-reproductive ramets. Key plant traits for the selection of oviposition sites by 31 butterflies, such as foliar water and chlorophyll contents, varied significantly across the open-32 closed ecotones. Foliar water content was always lower in the open microhabitats, whereas 33 foliar chlorophyll gradients differed between sun-and shade-adapted plants. The oviposition 34 behavior of Pieris butterflies across the ecotones differed significantly between the 35 thermotolerant species (P. rapae, preferentially selecting open microhabitats) and the 36 thermosensitive species (P. napi, selecting microhabitats protected by vegetation cover), 37 matching the values of thermal susceptibility estimated from parallel heat tolerance assays of 38 the populations. The larvae of the thermotolerant Pieris species grew under completely different 39 thermal conditions due to differential microhabitat selection, indicating marked interspecific 40 differences in thermal exposure (5-10 °C). These results suggest that the impacts of global 41 warming in these communities will likely be mediated by open-closed ecotones, which 42 determine pronounced local variability in thermal exposure, oviposition placement, and host-43 plant traits affecting larval performance in summer. 44 45