Forrest, Jessica R KCross, ReganCaraDonna, Paul J2021-04-142021-04-1420190003-0147https://www.journals.uchicago.edu/doi/abs/10.1086/701826http://hdl.handle.net/10393/41995https://doi.org/10.20381/ruor-26217Organisms must often make developmental decisions without complete information about future conditions. This uncertainty-for example, about the duration of conditions favorable for growth-can favor bet-hedging strategies. Here, we investigated the causes of life cycle variation in Osmia iridis, a bee exhibiting a possible bet-hedging strategy with co-occurring 1- and 2-year life cycles. One-year bees reach adulthood quickly but die if they fail to complete pupation before winter; 2-year bees adopt a low-risk, low-reward strategy of postponing pupation until the second summer. We reared larval bees in incubators in various experimental conditions and found that warmer-but not longer-summers and early birthdates increased the frequency of 1-year life cycles. Using in situ temperature measurements and developmental trajectories of laboratory- and field-reared bees, we estimated degree-days required to reach adulthood in a single year. Local long-term (1950-2015) climate records reveal that this heat requirement is met in only ∼7% of summers, suggesting that the observed distribution of life cycles is adaptive. Warming summers will likely decrease average generation times in these populations. Nevertheless, survival of bees attempting 1-year life cycles-particularly those developing from late-laid eggs-will be <100%; consequently, we expect the life cycle polymorphism to persist.enMegachilidaeadaptive plasticitybet hedgingcohort splittingthermal timevoltinismAltitudeAnimalsBeesClimate ChangeFemaleMaleModels, BiologicalSeasonsTemperatureAdaptation, BiologicalLife History TraitsArticle10.1086/701826