Heatwave impacts on immature stages of the solitary bee Osmia lignaria

Heat waves are an increasingly common aspect of global climate affecting individuals, populations and even species distributions. For many insects, including solitary bees, immature stages may be especially vulnerable because their limited mobility prevents them from seeking thermal refugia. Understanding their responses is also complex because development occurs across seasons, so developmental stages are exposed to different absolute heat wave temperatures. For spring bees, pupae and quiescent adults that develop during summer may be more robust to high temperatures than early larval stages that develop in spring. On the other hand, maximum temperatures of summer heat waves are greater than those of spring. To disentangle these effects, we applied experimental heat waves of different severity to early larval and pupal stages of the solitary bee Osmia lignaria. Naturally provisioned offspring were exposed to experimental heat waves of moderate or extreme temperature, with absolute temperatures adjusted for the season when each stage occurs. Eggs and early-instar larvae, which are present in late spring, experienced 31C or 37C heat waves, whereas pupa, which metamorphose in mid-summer, experienced 38C or 42C heat waves. We assessed survival immediately following heat waves and to emergence the following spring. Extreme heat during early development caused ~80% mortality, mostly during and immediately following the heat wave. Extreme heat during pupation caused over 60% mortality mostly occurring over the winter. Effects of moderate heat waves in spring were not different from controls, whereas in summer they were significantly greater suggesting that absolute temperature plays a role