Does nonreproductive swarming adapt to pathogens?

Honey bees play a vital role in the pollination of crops, fruits, and wild plants [1]. However, pathogens and parasites pose major threats to their fitness and survival due to the large number of declines reported recently in North America and several European countries [2]. Aside from the innate immunity of honey bees, indirect fitness benefits resulting from altered behavior may provide resistance against pathogens and parasites [3]. Although the study of vertebrate behavior has received increasing attention from researchers [4], the role of behavior in insect immunity has been much less appreciated. A recent publication regarding the incidence of pathogens in honey bee colonies and honey bee swarming behavior has positioned the honey bee at the forefront of efforts to understand the relationship between pathogens and complex behaviors [5] such as warning their nestmates of danger from predators [6]. Honey bee swarming, also termed reproductive swarming, is a natural division of the hive population and colony fission; when the number of workers exceeds the capacity of the hive, the old queen will travel to a new location with approximately three-fourths of the colony’s worker bees and drones. Generally, this swarming occurs between 10 AM and 2 PM on a warm sunny day in the spring or summer, seasons with sufficient plant nectar and temperate climate to facilitate the development and growth of the new colony [7]. Due to rapid growth, reproductive swarming is also useful for increasing the population and maintaining a high level of reproduction during times of rich food resources, and it is the major way of reproduction of honey bee colonies [8]. By contrast, however, nonreproductive swarming does not typically occur in spring or summer but in seasons in which there is insufficient plant nectar or when the hive has enough space to accommodate more honey bees. Nonreproductive swarming is mainly a resistance mechanism at the colony level [9]. Compared to reproductive swarming, incidents of nonreproductive swarming of honey bee colonies were found to be increased after July, which is not a reproductive season (S1 and S2 Movies). Therefore, these findings demonstrated that nonreproductive swarming as a collective behavior might have an additive function for colony survival or health. More importantly, honey bees cannot reproduce or ensure healthy hive development if they are unable to ensure their survival first [10]. Within this context, the relevance between nonreproductive swarming and population health is especially important. This article will focus on the relationship between nonreproductive swarming and pathogens and try to understand the possible functions of this collective behavior.