Eggs of the bee Osmia lignaria endure weeks of prolonged cold weather

Our insights into the responses of immature stages of vernal solitary bees to the thermal challenges of spring weather mostly come from studies of cavity-nesting megachilid bees in constant temperature incubators. Most of these studies reported either the average timing of adult emergence or the general pace of immature development. In this study, we tracked the longevity and developmental fates of eggs laid by nearly 200 female Osmia lignaria (Hymenoptera: Megachilidae) occupying observation nests set outdoors in a nature preserve. In the spring of 2022, the nesting population was challenged by a prolonged period of unseasonably chilly wet weather. As a consequence, fewer nest cells were provisioned, and hatching among the 2416 eggs was delayed for as much as a month. Surprisingly, these delays incurred no ill-effects on either egg survival or subsequent larval development, both of which were comparable to the much warmer spring of 2021 (3630 eggs). The population’s 33% reproductive shortfall in 2022 was instead attributable to limited hours warm enough for flight and foraging, in part evidenced by the paucity of multi-cell days. Eggs and larvae of this bee, and probably other vernal solitary bees, readily tolerate prolonged episodes of cold springtime weather.