Climate change had been linked to geographic, genotypic, phenological and population size shifts in a variety of insect species. However, little is known about the cumulative effects of these changes on community composition and the ecosystem services provided or affected by insects. Furthermore, we have limited abilities to predict changes because we have yet to gather sufficient empirical evidence on insect responses to climate warming across insects’ geographic ranges. In this study, we experimentally manipulated temperature in the field to investigate the response of eastern US ant communities to warming in southern (Duke Forest) and northern (Harvard Forest) locales. In addition to ant assemblage composition, we quantified foraging activity across temperature treatments because the foraging activities of ants provide important ecosystem services such as seed dispersal and nutrient cycling. We found that ant species diversity, abundance of the majority of ant species and foraging activity decreased with increased temperatures at Duke Forest. Some of these changes were associated with the increase of a single ant species, Crematogaster lineolata. At Harvard Forest, the abundance of most ant species declined under warmer temperatures, but ant species diversity was not affected by temperature and temperature effects on foraging activity were weak. Ant foraging activity was strongly associated with diversity rather than temperature at Harvard Forest. Our findings suggest that diversity and ecosystem services provided by ants will decline in warmer locales under climate warming. At cooler sites, the effects of warming may be less pronounced and therefore less beneficial to ants and other insects than expected under the current climate change paradigm. Together, these findings suggest that the ecosystem services provided by ants may be diminished across large spatial scales.