Likelihood of same diapause fate for nest-adjacent siblings of alfalfa leafcutting bees (Megachile rotundata, Hymenoptera: Megachilidae)

Megachile rotundata, the alfalfa leafcutting bee, is a managed solitary bee used to pollinate alfalfa for seed production in North America. These bees exhibit a unique life cycle where an individual either completes development to adulthood in summer without a diapause period or enters diapause as a prepupa, which is the overwintering life stage. Although various external factors are shown to influence diapause outcomes, the underlying mechanisms that determine these two developmental pathways are unclear. To better study when and how diapause outcomes are determined in immature bees, we investigated the likelihood that siblings occupying adjacent cells in their nest would experience the same diapause fate. We collected nests from commercial bee aggregations in alfalfa fields. The first two cells were excised from hundreds of nests collected on the same day, and only cells containing eggs or first instars were retained and then monitored over time. We recorded whether an offspring developed directly to the adult stage or remained in the prepupal stage through the fall for overwintering. Our findings revealed that across the nesting season, 91% of cell-adjacent siblings had the same diapause fate. However, time of year was a factor in how likely these siblings were to have the same fate. Near the summer solstice, only 76% of adjacent siblings had a matched diapause fate, whereas several weeks before or after this period, siblings’ fates were nearly 100% matched. Additionally, egg to prepupa development for non-diapausing offspring was about five days faster than for diapausing offspring.