Honey bee virus causes context-dependent changes in host social behavior Amy C. Geffrea,1,2, Tim Gernatb,c,1, Gyan P. Harwoodd, Beryl M. Jonese, Deisy Morselli Gysic,3, Adam R. Hamiltonb, Bryony C. Bonningf, Amy L. Totha,g, Gene E. Robinsonb,d,h, and Adam G. Dolezald,4 aDepartment of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, IA 50011; bCarl R. Woese Institute for Genomic Biology, University of Illinois at Urbana–Champaign, Urbana, IL 61801; cFaculty of Mathematics and Computer Science, Department of Computer Science, University of Leipzig, 04109 Leipzig, Germany; dDepartment of Entomology, University of Illinois at Urbana–Champaign, Urbana, IL 61801; eProgram in Ecology, Evolution, and Conservation Biology, University of Illinois, Urbana–Champaign, IL 61801; fDepartment of Entomology and Nematology, University of Florida, Gainesville, FL 32611; gDepartment of Entomology, Iowa State University, Ames, IA 50011; and hNeuroscience Program, University of Illinois at Urbana–Champaign, Urbana, IL 61801 Edited by Joan E. Strassmann, Washington University in St. Louis, St. Louis, MO, and approved March 16, 2020 (received for review February 6, 2020) Anthropogenic changes create evolutionarily novel environments Varroa-vectored viruses (11) are not well understood. In ad- that present opportunities for emerging diseases, potentially dition, it is not known whether pathogen manipulation of host changing the balance between host and pathogen. Honey bees behavior is involved in disease transmission. Honey bees and provide essential pollination services, but intensification and other highly social insects present exceptional opportunities for globalization of honey bee management has coincided with parasites and pathogens, as tens of thousands of highly related increased pathogen pressure, primarily due to a parasitic mite/ individuals live together in densely populated colonies (12). To virus complex. Here, we investigated how honey bee individual combat the challenges posed by these factors, many social in- and group phenotypes are altered by a virus of concern, Israeli sects exhibit adaptive behaviors that reduce pathogen spread acute paralysis virus (IAPV). Using automated and manual behav- within their colonies (12). Transmission between colonies is ioral monitoring of IAPV-inoculated individuals, we find evidence also made difficult for pathogens by sophisticated recognition for pathogen manipulation of worker behavior by IAPV, and re- systems that help prevent foreign individuals from entering the veal that this effect depends on social context; that is, within colony (14). However, anthropogenic changes that increase the versus between colony interactions. Experimental inoculation re- density of colonies can disrupt these pathogen defense systems ECOLOGY duced social contacts between honey bee colony members, sug- gesting an adaptive host social immune response to diminish and increase pest and pathogen movement (8), and have been transmission. Parallel analyses with double-stranded RNA (dsRNA)- hypothesized to lead to increased intercolony virulence (6, 8, immunostimulated bees revealed these behaviors are part of a gen- eralized social immune defensive response. Conversely, inoculated Significance bees presented to groups of bees from other colonies experienced reduced aggression compared with dsRNA-immunostimulated bees, The increase in human population has fueled demand for pol- facilitating entry into susceptible colonies. This reduction was asso- lination services; the resulting intensification and globalization ciated with a shift in cuticular hydrocarbons, the chemical signatures of honey bee management has coincided with increased used by bees to discriminate colony members from intruders. These pathogen pressure. We hypothesized that Israeli acute paral- responses were specific to IAPV infection, suggestive of pathogen ysis virus (IAPV) can alter host social behavior, predicting dif- manipulation of the host. Emerging bee pathogens may thus shape ferent behavioral changes depending on social context. Using host phenotypes to increase transmission, a strategy especially well- automated and manual behavioral monitoring, we find that suited to the unnaturally high colony densities of modern apicul- honey bees have social immune mechanisms that may keep ture. These findings demonstrate how anthropogenic changes could IAPV from spreading within a colony, but IAPV infection results affect arms races between human-managed hosts and their patho- in behavioral and physiological changes that could increase gens to potentially affect global food security. transmission between colonies. These results show how IAPV could take advantage of modern apiculture to increase its vir- honey bee | virus | host–pathogen evolution | pathogen manipulation ulence and highlight the critical need to understand how hu- man manipulation of managed species can lead to increased nthropogenic changes create evolutionarily novel environ- pathogen pressure. Aments that present opportunities for emerging diseases (1, 2), potentially changing the balance of host–pathogen co- Author contributions: A.C.G., T.G., G.P.H., B.C.B., A.L.T., G.E.R., and A.G.D. designed re- search; A.C.G., T.G., G.P.H., B.M.J., A.R.H., and A.G.D performed research; A.C.G., T.G., evolutionary arms races (3, 4). This problem is likely to worsen as G.P.H., D.M.G., A.L.T., G.E.R., and A.G.D. analyzed data; and A.C.G., T.G., A.L.T., G.E.R., humans continue to move domesticated species around the and A.G.D. wrote the paper. world and adopt increasingly intense management practices. The The authors declare no competing interest. western honey bee (Apis mellifera), one of the world’s most im- This article is a PNAS Direct Submission. portant pollinators, has experienced this shift to increasingly Published under the PNAS license. industrial-scale apicultural practices over the past several decades Data deposition: All data have been deposited in the Dryad repository at https://doi.org/ (5–7), which involve moving honey bee colonies extensively and 10.5061/dryad.m63xsj3z8. maintaining them at unnaturally high densities (6, 8, 9) for polli- 1A.C.G and T.G. contributed equally to this work. nation of many important food and fiber crops (10). Increased 2Present address: Division of Biological Sciences, Section of Ecology, Behavior, and Evolu- pollination demand has also coincided with the worldwide spread tion, University of California, San Diego, CA 92093. of the parasitic mite Varroa destructor and several mite-vectored 3Present address: Center for Complex Network Research, Northeastern University, Boston, pathogenic viruses in the last 30 y, causing devastating losses to MA 02115. managed and wild honey bees (11). 4To whom correspondence may be addressed. Email: [email protected]. Although there is evidence for both individual-level and group- This article contains supporting information online at https://www.pnas.org/lookup/suppl/ level behavioral immune responses to combat other pathogens in doi:10.1073/pnas.2002268117/-/DCSupplemental. honey bees (12, 13), responses to the highly detrimental www.pnas.org/cgi/doi/10.1073/pnas.2002268117 PNAS Latest Articles | 1of8 Downloaded by guest on October 1, 2021 15). Unnaturally high apiary densities may create new oppor- characterized how virus treatment affects CHC profiles. We tunities for selection on traits related to increased pathogen aimed to understand whether changes in these behaviors and transmission (6, 12). physiological factors have the potential to allow pathogen ex- Here, we combined automated and manual behavioral moni- ploitation of the overcrowded conditions of human-managed toring with chemical analysis to test how honey bees respond to bee colonies. infection with Israeli acute paralysis virus (IAPV). IAPV is a Varroa-vectored (11) virus with the potential to evolve rapidly Results and Discussion (16). It is frequently found in honey bee colonies and is associ- In experiments with three colonies that were each continuously ated with increased colony mortality (11, 17, 18). Because many observed for 5 d, we found that inoculated bees engaged in honey bee pathogens, including IAPV (17), are also spread significantly fewer trophallaxis interactions with nestmates than horizontally through physical contact, we hypothesized that if did controls (33) (Fig. 1A; general linear mixed model [GLMM], honey bees have evolved adaptive behavioral responses to min- P < 0.0004). Reduced social interactions were not driven by an imize disease impact, experimental inoculation will elicit be- overall reduction in activity, as inoculated bees moved more haviors that decrease social interactions among colony (Fig. 1B; GLMM, P < 0.003). These results highlight the value of members (19). an automated monitoring system that measures specific behav- To test this prediction, we employed a recently developed iors in addition to general movement (Fig. 1C). We suggest that automated behavior monitoring system in which all colony the observed reduction in trophallaxis serves as an adaptive so- members are individually barcoded and tracked continuously cial immunity mechanism to reduce pathogen transmission via (20). This system features computer vision algorithms for physical contact within colonies, as has been hypothesized in detecting specific behaviors and allows for monitoring and other systems (12, 34). Honey bee viruses (35), and IAPV in quantification of movement, survivorship, and social food- particular (36),