Determining the mechanism of honey bee (Apis mellifera) self-removal behavior as a potential social immune response

The honey bee (Apis mellifera) is an economically important pollinator and a tractable system for studying the behavioral consequences of eusociality. As eusocial insects, honey bees live in colonies comprised of thousands of sterile female workers and only one reproductively active queen. Therefore, a worker’s inclusive fitness is best served by acting in the interest of her colony, even if that behavior curtails her own lifespan. Stressed bees typically leave the colony to forage early, which leads them to be underdeveloped and unproductive foragers. This precocious foraging behavior can even lead to colony collapse. In this study, we test the hypothesis that developmentally stressed young worker bees, that are too young to forage, remove themselves from their colony due to extreme stress. To confirm whether this behavior is a reaction to severe stress or an altruistic social immune response, we stressed bees with either temperature stress or parasitization by Varroa destructor mites during pupation. Bees that were stressed by either treatment (temperature shock or mite parasitization), as well as untreated controls bees, were tagged upon emergence and introduced to a common observation hive. We took daily attendance of focal bees and checked a trap at the hive entrance for self-removing bees every hour. We found that the stressed bees from both groups self-remove at a significantly higher rate than age-matched unstressed bees. Stressed bees also had smaller hypopharyngeal glands than unstressed control bees, indicating that this is a stress-driven behavior and a newly discovered form of premature exit.