Conflicting selection on workers underlies global honey bee colony losses

Abstract

There is clear evidence for global and unsustainable high losses of the managed Western honey bee, Apis mellifera, colonies raising major concerns about the sustainability of pollination services and food security. Here, we address a conflicting selection scenario in workers to better understand life-history trade-offs underlying colony losses. Due to developmental plasticity, annual fluctuations in selection pressures result in long-living winter bees and short-living summer bees as adaptive responses to lack of food and ceased brood rearing in winter, or ample food and rapid brood turnover in spring and summer. Since trait evolution is governed by a balance of conflicting fitness advantages, trade-offs between longevity and other traits are inevitable (e.g. immune defense or detoxification). As worker longevity is essential to ensure colony functionality, these trade-offs render long-living workers more susceptible to the same stressors compared to short-lived ones. It seems as if trade-offs in workers play a previously overlooked key role, and the incorporation of long-living workers in research efforts is long overdue if our aim is to effectively mitigate declines and losses of social insect colonies globally. Indeed, managed honey bee colony losses in temperate regions predominantly occur throughout winter, but research efforts are almost exclusively conducted with short-lived summer bees, thereby sustaining the danger of false negative results. Thus, we should not shy away from meeting the respective logistic challenges. Studies of conflicting selection on workers will be informative about mechanisms underlying life history trade-offs, thereby fostering conservation efforts in the social insects.