LITMIR.BIZ

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seems correct to classify a group of organisms as a superorganism when the organisms form a cooperative unit to propagate their genes, just as we classify a group of cells as an organism when the cells form a cooperative unit to propagate their genes” writes Tom Seeley (1989). Now let's turn our attention to the marvelous ways in which honey bees work together as a cooperative unit to maintain a healthy organism.

Part 2: Social Immunity: Bees as Their Own Doctors!

Group living in insects with its consequent division of labor, cooperative care of brood, and the overlap of more than a single generation in time and space are the hallmarks of the superorganism. Insects living within a coordinated framework, where tasks are divided among different bee castes and communication networks are compartmentalized in a confined space, are susceptible to the spread of disease from one individual to another. Likewise, their strict control of the nest cavity environment necessary to maintain the stable temperatures for brood care can be compared to a pathogen incubator. The group living of honey bees predisposes the individuals and the entire organism to epidemics. Fortunately, honey bees and other social insects have evolved highly adaptive behaviors that range from “constitutive” (aka prophylactic) to “inducible” (aka activated) responses that help prevent disease (Simone‐Finstrom 2017). Behaviors that reduce or eliminate pathogen exposure or pest infestation at the level of the superorganism are collectively known as social immunity.

      One of the advantages of a social (or group) response to preventing or actively eliminating an infection by a parasite or pathogen in honey bee(s) is a coordinated response from the colony. By doing so, the individual bee is able to conserve resources that it would otherwise expend on maintaining and delivering an individual response. The immune function of individual honey bees is costly and expressed to a lesser degree than in asocial insects; indeed, the mapping of the Apis mellifera genome revealed a surprising lack of immune specific genes (Evans and Pettis 2005; Simone et al. 2009). This does not mean that individual honey bees lack discrete methods for disease protection entirely. Like other insects, honey bees have a hard chitinous exoskeleton that protects against pathogen entry, possess hemocytes that can phagocytize foreign invaders (though they lack memory cells and any ability to produce protective antibodies like vertebrates), remove themselves from the colony when sick or dying, recruit specialized members to perform dangerous biosecurity tasks as guards and undertakers, and even mummify pests too large to carry out of the hive.

      In his comprehensive 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

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