Impacts of heat waves on solitary bee (Osmia lignaria) fitness and development: A comparative study of three western populations

Climate change is a major driver of global species declines, and heatwaves are particularly concerning because they are becoming more frequent and intense and can suddenly push organisms beyond their thermal limits. Cavity-nesting solitary bees may be especially vulnerable, as they develop immobile within above-ground nests with limited protection from extreme heat. Many bee species also span broad geographic ranges, experiencing diverse local climates that may shape their thermal tolerances; yet, we still lack a clear understanding of how heatwaves impact bee fitness across life stages and populations. To address this gap, we exposed developing Osmia lignaria from three western U.S. populations to experimental heatwaves during larval development. We assessed survival and development during and after the heat wave, and the following spring we evaluated adult outcomes, including emergence timing, ovary maturation, and thermal tolerance. Heat wave exposure during larval development reduced survival and altered developmental timing, with variation among populations in the magnitude of these effects. These findings offer insight into the vulnerability of solitary bees to heat waves and highlight the importance of life stage and geographic origin in shaping thermal resilience. Understanding these patterns is critical for predicting the effects of future climate extremes on bee persistence and pollination services in natural and managed ecosystems.