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In my third installment in a mini-series on social immunity I want to give a sum-mary of what this knowledge can teach

us about practical honey bee health manage-ment. It’s useful to remind ourselves that agriculture, by definition, is the manage-ment of food plants and animals to achieve unnaturally high productivity. For example, a typical Holstein cow produces 9 gallons of milk per day1, whereas her 90-lb calf re-quires only 4.5 quarts per day2, an amount equal to only 12.5% of her mother’s daily production. This 87.5% excess over bio-logical requirements is a good example of what agriculture does. And the same thing happens in beekeeping. A typical wild bee colony in autumn, after spending a summer converting incoming nectar into beeswax comb and workers, will end up with the balance of its nectar dehydrated down to about 44 pounds of honey, constituting its winter stores.3 If we think of this as analo-gous to the colony’s “surplus” honey, we may compare this value to colony yields for a commercial beekeeper which often range 2-5 times as high. I state this up-front

to reinforce the fact that our standards for productivity are inflated by our agricul-tural inputs which include swarm control, stimulative feeding, larger-than-natural nest cavities, selective breeding, and short-term disease and parasite relief with antibiotics and acaricides. Being open to lessons from evolution and epidemiology requires of us the ability to critique the reigning paradigm.

Drawing upon my last two installments, I will focus on the five levels of increasing engagement that an organism, or superor-ganism, uses to avoid or contain pathogens or parasites4: (1) preventing parasite uptake, (2) preventing parasite intake, (3) prevent-ing parasite establishment, (4) preventing parasite spread between tissues (or colony members), and (5) preventing spread of parasite to offspring.

1. Preventing uptakeThis is the first and most passive level of

resistance – avoiding colony member ex-posure to pathogens and parasites. This, I believe, is one of the main selection drivers for large inter-colony distances in nature,

known to range from 304-4848 meters5, another equally plausible reason being the avoidance of inter-colony food competition.

Epidemiology is rife with evidence that high host density encourages parasite vir-ulence. From the parasite’s perspective, success is all about reproduction, and by definition a parasite is an organism that reproduces at the expense of its host. If it were a benign relationship we would call it commensalism, and if it were mutually dependent we would call it symbiosis. So it’s no surprise that a parasite’s reproduc-tive success is usually positively associated with its virulence – the more reproductively successful the parasite, the more damaging to its host. This is likely true with Varroa and Apis mellifera, although many factors contribute to mite virulence, and it’s not easy to draw an equivocal connection be-tween mite reproductive rate and colony

What Social Immunity can Teach us about Honey Bee Health Management

Figure 1. Many hive designs around the world employ smaller entrance sizes than we typically do in the U.S. This is probably a good accommoda-tion to biology. in nature14, 70% of honey bee nest entrances are smaller than 40 cm2, and modal values (most frequently occurring values) range from 10-20 cm2. in contrast, the en-trance void in a typical 10-frame American hive is easily twice as big – 80 cm2. The hives shown here with comparatively small hive entrances are (l to R) from Poland, Azerbaijan, and northern ireland.

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morbidity.6 What is clear is that Varroa numbers do increase in conditions of high colony density. My PhD student Brett Nolan replicated apiaries of two colonies each with three levels of inter-colony dis-tance: 0 meters, 10 meters, or 100 meters. All colonies were rendered nearly mite-free with powdered sugar and acaricides, then one colony in each pair was inoculated with 300 Varroa mites. Four months later, apiary mite averages were significantly lower in the 100-meter apiaries.5 Obviously, it is not practical to advise beekeepers to separate their hives by 100 meters, but the principle still holds, and I encourage beekeepers to think about ways one can reduce colony densities in one’s operation.

A second passive defense measure is the bees’ natural propensity to restrict foraging to the oldest worker cohorts in the colony. Given that nest enemies are, by definition, outside the nest, it is that portion of the nest population that leaves the nest that is in danger of making initial contact with in-fective agents, or “propagules.” If old indi-viduals do pick up parasites, their relatively brief remaining lifetime limits their oppor-tunity to spread the parasites to nestmates. This age bias normally happens on its own, but there are management situations where the beekeeper may disrupt this pattern. For example, if the beekeeper makes up splits in the middle of the day when old bees are out foraging, the splits may end up with un-naturally high ratios of young bees. When a colony loses its old bees, young bees are able to precociously jump ahead to forag-ing behaviors.7 Because these precocious foragers have a lot of life left ahead of them, they have a higher than average like-

lihood of encountering parasites and living long enough to transfer them to nestmates. One way to reduce this risk is to restrict split making to the early evening hours so that the resulting splits end up with more natural age distributions.

2. Preventing intakeThe most practical way to restrict para-

site intake is to restrict nest entrance size. There is some compromise at work here because a generous entrance is advanta-geous for nest cooling in hot temperatures and for encouraging active foraging, but for the balance of the year a smaller entrance is advantageous for restricting the entry of parasites, robbers, and other nest enemies. Hive designs in many countries employ en-trance sizes roughly half the size of Ameri-can designs (Fig. 1), and I think this is a wise accommodation to biology.

3. Preventing establishmentIf a parasite evades the first two modes

of restriction, then the honey bee superor-ganism will try to prevent its establishment throughout the nest. For beekeeping, the most practical way to enhance this natural defense is to encourage the collection of propolis. For much of the history of this country there has been a bias against propo-lis, as beekeepers have tended to see it as an inconvenience for handling hive parts. When I was very young I remember queen bee advertisements listing “low propolis” alongside “gentle” and “big honey produc-ers.” We now know that this bias is mis-guided. Propolis not only has antimicrobial properties throughout the nest, but exposure to propolis turns down an energetically-

costly immune reaction in worker bees.8 There is evidence that workers use it to en-tomb certain hive invaders.9 It is unknown whether genetically selecting for propolis-hoarding behavior gives an economic benefit to the colony, but we do know we should not be selecting against it. It would be interest-ing to see if other management practices to stimulate propolis hoarding would be ben-eficial. “Roughing up” the interior of hive parts by using un-planed wood comes to mind. Similarly, exposure of screens inside hive interiors often prompts bees to plug the gaps with propolis (Fig. 2).

4. Preventing spread between member groups

If a parasite becomes established in the nest, then the superorganism attempts to limit its spread throughout the population. It is at this juncture that most bee breed-ing efforts at parasite and disease resistance are applied. Hygienic bees that are selected to detect and remove mite- and disease-infected brood10; bees selected to groom mites off themselves and nestmates11; and the USDA Russian strain of bees12 are three of the most well-known lines bred for dis-ease or parasite resistance. It makes sense for beekeepers to use these lines to the extent possible. Their phenotypes can be expected to have a ceaseless downward effect on mite spread and population growth in a colony.

Another strategy for restricting spread among groups is the bees’ natural tendency to segregate themselves into age-similar co-horts. The queen and younger bees tend to concentrate in the nest center over the brood while older bees congregate in the nest pe-rimeter. Bee-to-bee contact is more common within, rather than across, these compart-ments. This is an argument for taking care to retain the basic natural nest configuration with frames of brood central and contiguous to one another, and honey and pollen combs to the periphery.

Given that individual bees are often the source of infective propagules, it makes sense for beekeepers to minimize squashing bees during hive manipulations. Imagine the health concerns in human society if our daily routines included regular contact with dead and mangled human corpses! Yet, this is exactly what a careless beekeeper inflicts on his or her bees if one heedlessly pushes frames around and mashes bees under supers and lids. To speak nothing of the moral and ethical offense! Mashed bees release patho-gens and invite rapid disease spread.

5. Preventing spread to offspring swarmIn the context of honey bees, this strat-

egy speaks to parasite transfer from colony to swarm, and the best evidence for it is the tendency of infected workers to refrain from social contact with the queen.13 As it is the old queen who transfers with the swarm, this behavior serves to reduce the transmission of infective propagules to the new swarm. But in the context of beekeeping, this strat-egy is functionally irrelevant because swarm control is such an integral part of honey pro-

Figure 2. When bees are exposed to roughened hive interiors, whether un-planed wood or screen material (shown here), they often respond by coating the surface with propolis. it would be interesting to test if such physical stimuli do in fact increase propolis hoarding and, more importantly, its antimicrobial benefits.

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duction. But this also begs the question – to what extent should we be practicing swarm control at all? As this is bending close to my topic for next month, I will leave it for that discussion.

ConclusionThis month I have tried to extract prac-

tical clues for beekeeping from our limited knowledge of social immunity. Four of the 5 levels of engagement I talked about are easily translated into immediate beekeep-ing management. But disease biology also operates at larger scales of space and time, so next month I will take a broader look at the science of epidemiology – the science of disease spread and virulence evolution – and see what it has to say about practical honey bee health management.

References1 Holstein Association USA http://www.

holsteinusa.com/holstein_breed/ holstein101.html

2 Amaral-Phillips, D.M. et al. 2006. Feeding and managing baby calves from birth to 3 months of age. University of

Kentucky Cooperative Extension Ser-vice ASC-161

3 Seeley, T.D. 1995. The wisdom of the hive: The social physiology of honey bee colonies. Harvard University Press

4 Cremer, S. et al. 2007. Social immu-nity. Current Biology 17: R693-R702 DOI: 10.1016/j.cub.2007.06.008

5 from data cited in: Nolan, M.P. and k.S. Delaplane. 2016. Distance between honey bee Apis mellifera colonies regu-lates populations of Varroa destructor at a landscape scale. Apidologie DOI: 10.1007/s13592-016-0443-9

6 Correa-Marques, M.H. et al. 2003. Comparing data on the reproduction of Varroa destructor. Genetics and Mo-lecular Research 2(1): 1-6

7 Robinson, G.E. 1992. Regulation of di-vision of labor in social insect societies. Annual Review of Entomology 37: 637-665

8 Simone-Finstrom, M. and M. Spivak. 2010. Propolis and bee health: the natu-ral history and significance of resin use by honey bees. Apidologie 41: 295-311

9 Neumann, P. et al. 2001. Social encap-

sulation of beetle parasites by Cape hon-eybee colonies (Apis mellifera capensis Esch.). Naturwissenschaften 88: 214-216

10 Spivak, M. and G.S. Reuter. 1998. Per-formance of hygienic honey bee colonies in a commercial apiary. Apidologie 29: 291-302

11 Guzman-Novoa, E. et al. 2012. Ge-notypic variability and relationships between mite infestation levels, mite damage, grooming intensity, and re-moval of Varroa destructor mites in se-lected strains of worker honey bees (Apis mellifera L.). Journal of Invertebrate Pa-thology 110: 314-320

12 Rinderer, T.E. et al. 2001. Resistance to the parasitic mite Varroa destructor in honey bees from far-eastern Russia. Apidologie 32: 381-394

13 Wang, D.I. and F.E. Moeller. 1970. The division of labor and queen atten-dance behavior of Nosema-infected worker honey bees. Journal of Economic Entomology 63: 1539-1541

14 Seeley, T.D. and R.A. Morse. 1976. The nest of the honey bee (Apis mellifera L.). Insectes Sociaux 23(4): 495-512

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