Larval Honey Bee Response to Various Doses of Bacillus larvae Spores1

Abstract

Larvae from 4 sister queens of Apis mellifera L. from each of 2 genetically different lines of honey bees, 1 susceptible and 1 resistant, were subjected in 2 trials to 7 dosage levels of spores of Bacillus larvae White. Doses applied were 0, 20, 70, 130, 950, 11,000, and 114,000 spores in 0.287 mm3 of distilled water placed individually in the larval food. Larvae 0-6, 18-24, and 36-42 hours old were used to test effect of the spore levels on different ages. In the 0-6 hour age group, the median lethal dosage and slopes of the 2 lines were nearly the same. The susceptible line had an LD50 of 275 spores and a slope of 0.826 compared with an LD50 of 187 spores and a slope of 0.835 for the resistant line. There was a threshold below which larval mortality did not exceed the check, while any dose above 950 spores gave greater than 80% mortality. A 18-24 hours, the LD50''s were 2506 and 1300 spores, while the slopes were 0.496 and 0.663 for the resistant and susceptible lines, respectively. The resistant line had developed resistance much earlier, as is evidenced by the LD50 and slope. The 36 to 42-hour age group had attained sufficient resistance to resist infection at all dosage levels and no LD50 or slope could be ascertained. Neither line greatly exceeded the 10% mortality level at any spore concentration applied at this age. The conclusion was that dosage-mortality relationships in the Bacillus larvae-honey bee system are greatly influenced by larval age and genetic constitution.