> > As Allen points > > out taking colonies off treatment does not necessary result > > in an outbreak of AFB. My guess is that long term use of antibiotic should reduce the spore count. As spores become active the antibiotic present gets them. I guess there is some density of the bacteria that would need to be present in the gut of a larvae to kill it before it became an adult. If you stay below that level you don't start the "kill larvae"/"clean scale from cell"/"reinfect larvae" cycle with the house bees. Adding to the above: Bucher(1958) determined the LD50* of bacillus larvae to be 35 spores in one-day-old honey bee larvae. *LD 50 The dose(number of microorganisms) that will kill 50% of the animals(honey bee larvae) in a test series. Bob