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Summary
Bacillus larvae germinated, multiplied and formed endospores internally in honeybee pupae following spore injections into the cephalic haemocoel. The LD50 was approximately 5000 injected spores per pupa. Resistance mechanisms appeared to be present in pupae inoculated with low doses of spores, some of the spore-inoculated pupae completing their development and emerging as apparently healthy adults. Spores from four different sources were of equal virulence. Changes in body colour were observed as early as 12 hours after spore injections. Pupae inoculated one day after pupation were the best indicators of an infection, and they showed the typical tan to brown colour of American foul brood disease by the fourth day of pupal life.
The ease of inoculating pupae, combined with the sensitivity of the test, makes pupal inoculations a convenient and useful bioassay method for determining relative spore viability.