Differential susceptibility across honey bee colonies in larval chalkbrood resistance

Chalkbrood susceptibility of in vitro reared honey bee larvae was investigated. Larvae were grafted from 3-4 colonies headed by pure mated queens of Apis mellifera carnica, A. m. ligustica and A. m. mellifera, respectively. Three day old larvae were fed with different dosages of Ascophaera apis spores and a clear dose-response relationship was shown. Over the whole experiment LD50 estimates ranged from 55 to 905 spores. The response differed significantly (up to a factor ten) between colonies of the same subspecies. The mean time to death decreased with increased dose, with more larvae dying faster after eating more fungal spores. The A. m. ligustica larvae used in this study were less susceptible to A. apis than A. m. mellifera and A. m. carnica larvae. However due to the limited number of colonies included and the high variation shown we cannot predict that any A. m. ligustica colony is better adapted to cope with A. apis than colonies of A. m. carnica and A. m. mellifera.