Social fever or general immune response? Revisiting an example of social immunity in honey bees

Authors: Goblirsch, Michael; WARNER, JENNY - University Of Minnesota; SOMMERFELDT, BROOKE - University Of Minnesota; SPIVAK, MARLA - University Of Minnesota

Submitted to: Insects
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/11/2020
Publication Date: 8/13/2020
Citation: Goblirsch, M.J., Warner, J.F., Sommerfeldt, B.A., Spivak, M. 2020. Social fever or general immune response? Revisiting an example of social immunity in honey bees. Insects. 11(8):528. https://doi.org/10.3390/insects11080528.
DOI: https://doi.org/10.3390/insects11080528

Interpretive Summary: Honey bees are social organisms that may engage in a group-level response when threatened by parasites and pathogens. This group-level response against pathogens and parasites has been termed social immunity and involves the cumulative effort of some members of the group to limit the spread of parasites and pathogens to uninfected members of the group. Examples of social immunity in honey bees include hygienic behavior, or the removal of diseased brood, and the collection and deposition of antimicrobial plant resins (propolis) on interior surfaces of the hive. Advances in our understanding of another form of social immunity, social fever, have been lacking. Adult honey bees have been shown to raise the temperature of the hive in response to the fungal pathogen, Ascosphaera apis. Ascosphaera apis is the causative agent of chalkbrood disease, a common disease of honey bee colonies; it requires specific temperature and humidity conditions for infection and growth in immature bees. The increase in hive temperature by adult bees is thought to limit the spread of A. apis infection to uninfected brood. We established small hives and placed them in the laboratory under controlled conditions. We monitored the temperature of the brood area for 40 days. This observation period was broken into five distinct segments corresponding to hive manipulation that involved sucrose solution feeding: Pre-Feed, Feed I, Challenge, Feed II, and Post-Feed. Ascosphaera apis was administered to hives as a 1% solution of ground sporulating chalkbrood mummies in sucrose solution during the Challenge period. Like previous reports, we observed an increase in hive temperature during the Challenge period. However, all hives presented signs of chalkbrood disease, suggesting that elevation of the temperature was not sufficient to stop infection among immatures. We also began to explore the molecular mechanisms of temperature increase by exposing adult bees in cages to A. apis in the absence of confounding cues from immatures. Compared to adult bees given sucrose solution only, bees exposed to A. apis had increased expression of two antimicrobial peptides but expression of a heat and a general stress marker were unchanged. These results indicate that adult honey bees exposed to a brood pathogen elevate the temperature of the hive and initiate an immune response, but the effect of this fever on preventing disease requires further study.

Technical Abstract: Honey bees use several strategies to protect themselves and the colony from parasites and pathogens. In addition to individual immunity, social immunity involves the cumulative effort of some individuals to limit the spread of parasites and pathogens to uninfected nestmates. Examples of social immunity in honey bees that have received attention include hygienic behavior, or the removal of diseased brood, and the collection and deposition of antimicrobial resins (propolis) on interior nest surfaces. Advances in our understanding of another form of social immunity, social fever, have been lacking. Honey bees have been shown to raise the temperature of the nest in response to the temperature-sensitive brood pathogen, Ascosphaera apis. The increase in nest temperature (~0.6ºC) is thought to limit the spread of A. apis infection to uninfected immatures. We established observation hives and monitored the temperature of the brood nest for 40 days. This observation period was broken into five distinct segments corresponding to sucrose solution feedings: Pre-Feed, Feed I, Challenge, Feed II, and Post-Feed. Ascosphaera apis was administered to colonies as a 1% solution of ground sporulating chalkbrood mummies in 50% v/v sucrose solution during the Challenge period. Like previous reports, we observed a modest increase in brood nest temperature during the Challenge period. However, all hives presented signs of chalkbrood disease, suggesting that elevation of the nest temperature was not sufficient to stop the spread of infection among immatures. We also began to explore the molecular mechanisms of temperature increase by exposing adult bees in cages to A. apis without the presence of immatures. Compared to adult workers given sucrose solution only, workers exposed to A. apis had increased expression of the antimicrobial peptides abaecin (p = 0.07) and hymenoptaecin (p = 0.04), but expression of the heat shock response protein Hsp 70Ab (p = 0.76) and the nutritional marker vitellogenin (p = 0.72) were unaffected. These results indicate that adult honey bee workers exposed to a brood pathogen elevate the temperature of the brood nest and initiate an immune response, but the effect of this fever on preventing disease requires further study.