Most of our understanding of the effects of climate warming on insect body size comes from laboratory experiments. Whether these studies predict patterns in nature is largely unknown. Here we examine the relevance of laboratory warming experiments for wild populations of the butterfly Pieris rapae. We tested two predictions: (i) butterflies reared at warmer temperatures in the laboratory should attain smaller adult sizes and have reduced flight ability, and (ii) in nature, this trait combination should lead to smaller butterflies visiting fewer flowers and accumulating less pollen. Overall, we found that warm‐reared butterflies were indeed smaller and flew more slowly compared to colder‐reared conspecifics. Additionally, wild‐caught small butterflies carried fewer, and a lower diversity of pollen grains compared to larger butterflies. Our warming experiments thus largely predicted pollen collection patterns in wild P. rapae. This study demonstrates that increased temperatures will likely have important consequences for butterfly‐plant interactions in nature.