Progress of Paenibacillus larvae larvae infection in individually inoculated honey bee larvae reared singly in vitro, in micro colonies, or in full-size colonies

The progress of infection of American foul-brood (AFB), Paenibacillus larvae larvae, in honey bee (Apis mellifera) larvae, prepupae, and pupae were studied in in vitro rearings, micro colonies and full-size colonies. P. I. larvae spores in various known numbers (3-1384 spores per larva) were inoculated at a larval age of 24-28 h. The in vitro study suggested that survival time decreased with increasing spore inoculation dose. No larvae inoculated with three spores died before day 6, but 30% of the larvae inoculated with 1384 spores had died at day 4, 36 h before the time of capping tin a bee colony). On day 4, approximately 480 000 bacterial colonies were cultured per larvae inoculated with 1384 spores at a larval age of 24 h. Viable counts of Fl I. larvae per larvae from inoculation and four weeks onwards fitted (R-2 = 0.917) a standard model for bacterial growth: Iny = b In (1 + exp (a - rt)), where y is predicted viable count, r is growth rate, t is larval age in hours, and b and a are constants. Bacterial growth rate in the four weeks infection period was estimated to be r = 0.179+/-0.030 h(-1). In the queenless micro colonies and full-size colonies the first signs of AFB were not visible to human eyes until day 4. By day 3 nurse bees removed 40% and 50% of the inoculated larvae, respectively, indicating that they are able to detect infected larvae before disease symptoms are visible. An early removal behaviour probably is a very important trait to focus on when breeding for resistance against AFB. The removal behaviour of nurse bees in micro colonies was well correlated with removal in full-size colonies. We conclude that, the micro colonies may serve as a labour and time saving model for full-size colonies when testing the removal behaviour of selected bee lines.