Apis mellifera capensis larvae show low resistance to a highly virulent Paenibacillus larvae field strain

American foulbrood (AFB) poses a serious threat to honey bees worldwide. Clinical signs of AFB were first detected in South Africa in 2008 and are currently confined to the Western Cape, an area inhabited by A. m. capensis. The virulence of isolated AFB field strains and the resistance of A. m. capensis larvae to these field strains were investigated. We individually infected larvae in full-sized A. m. capensis colonies with different doses of the ERIC I, subtype Ab field strain of P. larvae identified from colonies in the Western Cape and assessed larval mortality over 14 days. We compared larval mortality in colonies from a previously AFB-exposed A. m. capensis population to larval mortality in colonies from a non-AFB exposed population. The field strain tested was highly virulent, with the LD50 found to be less than a single spore per larva in both populations, with there being no statistical difference between the populations in larval mortality rates. Our results indicate that once infected with AFB, both A. m. capensis populations responded similarly to the infection and the larvae from both populations showed low resistance to AFB. This indicates that the limited impact of AFB throughout Africa is not due to the direct resistance of larvae to AFB infection, but rather to other factors such as colony-level resistance resulting from highly developed hygienic behaviour, the dispersed nature of beekeeping in Africa, and the swarming/absconding nature intrinsic in African honey bees.