Mechanisms of parasite resistance in honey bees: Interactions among individual and social immune defenses

Honey bees and other pollinators are currently suffering from large-scale declines due to issues such as habitat loss and the emergence of widespread or possibly new parasites. Social insects have developed a suite of defenses to combat various stresses, both at the individual level (e.g., physiological defenses) and group level (e.g., multiple mating of queens, resin collection, hygienic behavior, grooming of nestmates). Given the fact that no clear single cause has been pin-pointed regarding global bee losses, understanding the combined roles of these defenses is of particular importance. Understanding the evolution of physiological immunity of individuals has been of great interest, particularly with the finding that honeybees have a reduced set of gene families dedicated to immunity as compared to other solitary insects. However since honeybees live in densely populated colonies of constantly interacting, genetically related individuals, we must also consider how parasites are transmitted and subsequently resisted at the group level. Much of these group-level defenses—known as social immunity—emerge from the collective behaviors of individuals that can effectively defend against parasite transmission and growth.   There is a large gap of information concerning how this suite of individual and social defenses that honeybees are able to employ interact to affect parasite transmission and colony fitness.  In particular effects of genetic diversity on colony health and productivity have been well documented, but it is currently unclear as to how increased genetic diversity leads to increased disease resistance. Since physiological immunity does have a genetic component in honeybees, it is possible that colonies composed of genetically diverse individuals experience reduced infection intensities due to differing susceptibility to parasites. On the other hand, task allocation and behavioral traits are also affected by genetic diversity and so genetically diverse colonies may show a wider range in mechanisms of social immunity.