Body size is related to fitness in many insects. In solitary bees offspring body size is largely determined by maternal provisions and microclimate. We studied the effect of quantity and quality of provisions and rearing temperatures (20, 25 and 30 • C) on body size in the Red Mason bee, Osmia bicornis. Cocoon weight increased with provision weight and decreased with increasing temperature. High temperature (30 • C) led to high mortality of larvae and smaller body size, partly due to incomplete consumption of the provisions. Diversity of pollen in the provisions was low, and most provisions primarily consisted of mainly oak and maple pollen. Pollen diversity did not affect cocoon weight and males provided with pure oak or pure maple pollen showed similar adult size. Our results suggest that the quantity of provisions and temperature, but not pollen quality influence the development of O. bicornis progeny and might affect population dynamics of this abundant European pollinator. Osmia bicornis / body size / pollen diversity / temperature

-Since the temperature during development may affect growth and fitness in insects, climate change might affect important life history traits of solitary bees. We investigated the impact of three fluctuating and three constant temperature regimes on prepupal weight, mortality, and development time of Osmia bicornis. Prepupal weight decreased with increasing temperature, but not as strong under fluctuating conditions. Adult mortality increased in the warm treatments. Fluctuating (versus constant) temperatures accelerated development in the most stages and temperature regimes. The duration of almost all developmental phases decreased with increasing temperature, except for the prepupal phase that was prolonged in the warm treatments. The differences in thermal responses to fluctuating vs. constant temperatures illustrated the importance of fluctuating temperatures in studies investigating potential consequences of climate change for insects, including pollinators.Osmia bicornis / temperature / development / climate change