Implications of horizontal and vertical pathogen transmission for honey bee epidemiology

The degree to which a disease evolves to be virulent depends, in part, on w hether the pathogen is transmitted horizontally or vertically. Eusocial ins ect colonies present a special case since the fitness of the pathogen depen ds not only on the ability to infect and spread between individuals within a colony, but also on the ability to spread to new individuals in other col onies. In honey bees, intercolony transmission of pathogens occurs horizont ally (by drifting or robbing) and vertically (through swarming). Vertical t ransmission is likely the most important route of pathogen infection of new colonies. Theory predicts that this should generally select for benign hos t-parasite relationships. Indeed, most honey bee diseases exhibit low virul ence. The only major exception is American foulbrood (AFB). In light of cur rent ideas in evolutionary epidemiology, we discuss the implications of hor izontal and vertical pathogen transmission for virulence of AFB and other h oney bee diseases.