Tolerance Mechanisms against American Foulbrood in Honey Bee Larvae and Colonies

Recently, increasing incidences of AFB and problems controlling the disease – based on resistance build-up in the bacteria, increased residue problems, and loss of natural resistance in bees have renewed the interest for AFB tolerant honey bee strains world-wide. The honey bee possesses different tolerance mechanisms against AFB – e.g. larval resistance, hygienic behaviour and food inhibition of bacterial growth. Up till now, in Denmark, the only method used by queen breeders in testing strains for AFB tolerance has been testing for degree of hygienic behaviour mainly by inserting killed capped brood and examining removal. Therefore, the aim of our study was to examine general hygienic behaviour regarding removal of both non-capped as well as capped larvae and larval tolerance against AFB in three different strains of honey bees. The ability to remove capped brood killed by freezing and removal of infected non-capped and capped brood was tested and correlated. Furthermore, the dose-response relationship of P. l. larvae was examined in 24-28 h old honey bee larvae of the same strains reared and inoculated in the laboratory. The results show that there were significant differences in removal behaviour and larval tolerance among the different strains. However, none of the strains removed all killed brood and there were no seasonal variations in the removal behaviour within the different strains. Some Danish Buckfast strains seem to have lost most of their tolerance mechanisms except hygienic behaviour. Our findings suggest that the hygienic behaviour expressed through removal of frozen capped brood alone or in combination with larval tolerance may not be sufficient to model whether a honey bee strain will be able to overcome an AFB infection.