Investigating hygienic behaviour and AFB resistance of Apis mellifera capensis colonies: are Cape honey bees hygienic and how well do they cope with the disease?

In South Africa, clinical signs of American foulbrood (AFB) are limited to the province of the Western Cape, infecting Apis mellifera capensis. Currently, colony-level virulence of AFB field strains and overall resistance of A. m. capensis colonies to AFB are unknown. We evaluated hygienic behaviour and colony AFB resistance in 15 full-sized A. m. capensis colonies. Eight colonies originated from a population previously exposed to AFB and seven colonies were from an AFB-unexposed population. Firstly, hygienic behaviour of all colonies was assessed by the freeze-killed brood assay. Twenty percent of all colonies removed more than 95% of frozen brood in 24 h, while a further 26.67% removed more than 95% of dead brood in 48 h. There were no differences between the two populations in the level of hygienic behaviour. Secondly, we infected the same A. m. capensis colonies with the ERIC I subtype Ab field strain of P. larvae with a dose of 90 CFU/larva. We monitored colonies for 6 months, quantifying brood production, brood patchiness, and the number of brood cells infected with AFB. The P. larvae field strain had high virulence for all colonies regardless of previous AFB exposure or level of hygienic behaviour. The disease progressed slower in the AFB-unexposed population than in the AFB-exposed population. Colonies that removed 95% or more dead larvae within 48 h showed lower proportions of AFB infection than colonies expressing poor hygienic behaviour, which suggests that hygienic A. m. capensis colonies are more resistant to AFB.