Climate change and intensifying agricultural practices are reshaping ecosystems through increased thermal variability and altered landscape-level crop composition. These shifts impose novel, potentially interacting selective pressures on agricultural pests. As ectothermic arthropods, pests such as the wheat curl mite (Aceria tosichella) possess physiological and life-history traits – most notably body size – that are tightly coupled with external environmental conditions. While temperature-induced plastic and evolutionary shifts in body size are well documented, the interactive effects of thermal variability and host-plant specialization remain poorly understood. This knowledge gap is particularly critical in intensively managed agroecosystems, where rapid environmental change may accelerate evolutionary trajectories. This study investigates how fluctuating versus constant biotic (cereal host) and abiotic (thermal regime) environments drive morphological adaptation in a globally significant cereal mite pest. We employed a replicated experimental evolution approach to disentangle the individual and interactive effects of host-plant and temperature variability. Mite populations, originating from either constant or alternating host- plant regimes, were exposed to one of three thermal regimes for 30 generations: constant, regularly fluctuating, or stochastically fluctuating, simulating key features of contemporary crop production systems and increasing climatic unpredictability. Morphological divergence was quantified by measuring female body length and width at two distinct time points. First, prior to experimental evolution, baseline morphological data were collected from 561 females (295 from constant and 266 from alternating host regimes) to ensure that the starting points were similar. Second, following the 30-generation selection period, 2472 females were measured across three thermal treatments. In total, 3033 mites were analyzed for length. Width was measured for a subset (468 baseline, 1920 experimental) where positioning permitted accurate dorsal imaging. This comprehensive dataset allows for a precise assessment of evolutionary change versus ancestral states. By explicitly manipulating both host-plant and thermal regimes, this study indicates that mite morphology may be influenced by host specialization. Emerging signals from the analysis of thermal regimes suggest that different levels of environmental variance favor different phenotypic optima. An interactive effect indicates that the history of host-plant specialization modulates the mites’ morphological response to thermal variability. These findings reveal how selection in complex, heterogeneous landscapes shape the evolutionary trajectories of a key morphological trait. By characterizing these adaptive responses in Aceria tosichella, our study provides a framework for predicting pest dynamics under changing climatic conditions, offering critical insights for the development of resilient pest management strategies in an era of increasing environmental stochasticity.