Susceptibility of the Asian honey bee, Apis cerana, to American foulbrood, Paenibacillus larvae larvae

American foulbrood (Paenibacillus larvae larvae), is a major concern of the beekeeping industry in Taiwan. However, disease signs have never been encountered during hive inspections of local colonies of the Asian honey bee, Apis cerana. To study the susceptibility of A. cerana larvae to AFB, various doses of P. I larvae spores were added to larval food and disease development was monitored. Results showed that 1-day-old larvae were most susceptible, next were 2-day-old larvae, while 3-day-old larvae showed no signs of disease even when fed a large dose (4.5 x 10(4) spores/larva). The negative correlation between susceptibility and larval age was similar to that found for A. mellifera. Further, at the susceptible age, A. cerana larvae showed higher resistance than A. mellifera larvae when fed the same dose of spores. The dose of spores that would cause 95% mortality of A. mellifera larvae only led to 47.1% mortality of A. cerana larvae of the same age. This resistance by A. cerana larvae apparently was not totally related to their innate immune capability. An important aspect contributing to the resistance of A. cerana was the fact that up to 82.2% of inoculated larvae were removed by adult workers before the capped stage. This adult hygienic behaviour effectively decreased the level of spore contamination inside the hive. In contrast, results showed that A. cerana pupae were more susceptible when vegetative cells of P. I. larvae were injected into the pupal haemocoel of both species of bees.