review of social immunity in honey bees, Simone‐Finstrom (2017) described the colony level adaptations for health in a continuum from prophylactic to activated: polyandry, task allocation, transfer of compounds and microbiota, resin use, allogrooming, hygienic behavior, social fever, and absconding. On the one extreme, diverse genes made possible by multiple matings and the compartmentalization of honey bee societies offer fixed preventative measures for health. The diversity that comes from numerous patrilines is linked closely to colony vigor and disease resistance and, once a queen mates, the colony's diversity (and thereby the protective alleles coding for disease protection) can only be changed by requeening. Likewise, the social structure of the honey bee colony, with its separation of castes, offers an important first line of defense against infectious disease since castes are separated in both time and space. Yet, the allocation of tasks is rarely altered by pathogen exposure.
On the other extreme, both social fever and absconding are actions taken by honey bees predominantly as a consequence of exposure to a pathogen and represent specific actions to combat the agent. Those social immune strategies located in‐between on the continuum may offer both prophylactic and treatment modalities; for example, the collection of resins can be preventative when bees seal their nest cavity in a complete protective “propolis envelope” or resin gathering can be activated by a specific pathogen as a kind of “self‐medication.” In our overview of social immunity, we will focus on just three of these traits: allocation of tasks with compartmentalization, use of compounds with antimicrobial actions – both bee‐derived and plant‐derived, and social fever. The miticidal actions of grooming and hygienic behavior are covered in detail elsewhere in this book on chapters about wild colony health, the biology of the varroa mite, and queen breeding for mite resistant honey bees.
Task Allocation and Compartmentalization
Group living elevates the risk of disease transmission through the close intermingling of thousands of individuals, especially for pathogens that are spread by direct contact. In eusocial organisms like the honey bee, the homogeneity in closely‐related individuals (all worker bees are daughters of the queen) together with the uniform physical environment both contribute to heightened risk of pathogen transmission. However, the complex social structure of honey bee colonies with its division of labor and allocation of tasks is one of the most important first levels of protection against disease (Cremer et al. 2007). In fact, the selection pressure of pathogens likely contributed to the evolution of social organization in honey bees (Naug and Camazine 2002; Stow et al. 2007). Modeling of honey bee societies depict a highly compartmentalized structure inside the hive with the core of the colony consisting of young bees surrounding a single queen with the foragers existing on the periphery. Even the dance stage of the foragers is located just inside the hive entrance so that the returning foragers – the bees most likely to bring novel parasites and pathogens from their travels outside the hive – are confined in a form of localized quarantine. The distribution of bees into castes with corresponding age classes, further serves to isolate potential spread of infection with young bees of the same age interacting regularly and overlapping spatially, while bees of different ages have limited direct contact (Baracchi and Cini 2014).
Naug and Camazine (2002) outline three key features of colony organization that may influence pathogen transmission in a group of social insects. Division of labor, interaction network, and colony demography collectively define the epidemiology of transmission. In division of labor, different groups of bees perform different tasks and these tasks are allocated based on the bee's morphology (physical polyethism) or their age (temporal polyethism). It is well known that honey bees conduct the safest jobs inside the hive first, followed by the riskiest jobs outside the hive (primarily defense, scouting and foraging) in the last part of their lives. While worker bees may skip tasks and perform more than a single task at each age, the general progression of tasks begins with cell cleaning, brood care, and tending of the queen, followed by comb building, handling of nectar and pollen, and finally guarding the entrance and foraging. This separation of duties serves to reduce the spread of pathogens in the colony. Honey bees that perform cell cleaning tasks or the hygienic behavior of removing infected brood are highly specialized and do not perform other tasks such as feeding larva that could transfer pathogens from infected to healthy larvae (Seeley 1982). The timing at which bees pass through the various age dependent work schedules in a honey bee colony could also profoundly impact the spread of pathogens and such timing can be altered by the honey bee itself! In infections with the protozoan Nosema apis and the sacbrood virus, honey bees have evolved a process known as precocious foraging. In these infections, honey bees move through the temporal schedule of working inside the hive in a fewer number of days and thus move more quickly to outside hive tasks. In this way, the spread of the protozoan and virus are slowed as the number of susceptible individual bees declines more quickly.
Figure 2.2 Trophyllaxis, or the transfer of food from bee to bee, augments disease transmission. Yet, the allocation of tasks across different castes and ages of bees together with separation of entire groups of bees across both space and time represents a sophisticated strategy for biosecurity in a honey bee colony.
In their modeling of disease dynamics within the confines of the social organization of a honey bee colony, Naug and Camazine (2002) observed that the separation of duties through discrete caste division was insufficient by itself to limit the transmission of a pathogen. Both the interaction networks and colony demography were found to be essential elements in the protective mechanism provided by social organization. Networks of interaction define the nature and frequency of contact among hive members and therefore influence the rate of pathogen transmission in a group of social insects. Frequent contact occurs during the transfer of food between individuals by trophyllaxis, a common occurrence that can augment the spread of disease through a honey bee colony (Figure 2.2). Brood diseases such as American foulbrood are transmitted by the passage of spores from worker bee to worker bee during trophyllaxis with subsequent infection of the larva during feeding by nurse bees. The cleaning of foreign material from the cuticle of another bee, or allogrooming, is a form of social immunity that helps remove mites and other pathogens from the exterior of individual bees. However, some viruses such as Chronic Bee Paralysis Virus seem to benefit from the activity, or may even exploit it, all in an effort to help spread the virus to other bees in the colony. Disease transmission is also influenced by colony demography–the size and density of the honey bee population. Big colonies are more likely to contact pathogens because of a larger workforce of foragers working outside the hive, and once a pathogen gains entrance, pathogen spread will occur faster in a dense colony where bee‐to‐bee interactions are more frequent (Naug and Camazine 2002).
Antimicrobial Compounds Produced by Bees
The recent discovery that bee venom, the collection of vasoactive peptides injected by worker bees using their stinger in defense of the colony, is found on the cuticle of worker bees as well as on the wax surface of the nest comb suggests that venom may play an antiseptic role in social immunity (Baracchi et al. 2011). Bee venom consists of a mix of biogenic amines, peptides, and proteins with neurotoxic action while also breaking down mast cells and stimulating the release of vasoactive substances. More recently, bee venom has been shown to have antimicrobial properties as well. Since there is a complete lack of venom peptides on the cuticle of drones and newly emerged bees, one can surmise that the venom found on the cuticle of worker bees is placed there by grooming behavior from the venom gland itself. Therefore, the allogrooming behavior of bees to remove pests and pathogens may be augmented by the defense provided by the neurotoxic peptides of bee venom (Figure 2.3).
Honey bees synthesize a variety of antimicrobial compounds in response to infection
