Oxytet remained effective for decades against AFB, until beekeepers started
applying it continuously as Crisco patties, at which time resistant AFB
bacteria became prevalent.  Could be a coincidence, or not.

TM was first used on bees in the 1950s. I encountered TM patties when I started beekeeping in 1974; they were already widespread. TM resistant AFB was not identified until the late 1990s. As a bee inspector from 2006-2008, I saw very little of it, and mostly of that was traceable to one outfit which sold off their bees. I asked Bart Smith if the info on incidence in the US was available. He said it had been collected but no one had ever compiled the data into a useful form. 

¶

> The most noteworthy contribution to the control of bee diseases has been the use of sulfathiazole, terramycin and other therapeutics in the food of bees in the spring and fall. By their use, at least one disease which had been considered as incurable for centuries can be prevented or controlled without the destruction of colonies or valuable equipment. The use of chemicals and antibiotics as therapeutic agents in disease control has measurably strengthened the position of the honeybee in our general economy and will save the beekeeping industry many thousands of dollars annually.

Eckert, J. E. (1955). Bee diseases as factors in the life and behavior of the honeybee colony. Insectes Sociaux, 2(3), 187-194.

¶

> Terramycin (Oxytetracycline HCl) was administered to honeybee colonies in a solid cake or patty. 

Wilson, W. T., Elliott, J. R., & Hitchcock, J. D. (1971). Antibiotic extender patties for control of American foul brood. Journal of Apicultural Research, 10(3), 143-147.

¶

> We suspected that AFB levels rose along with the emergence of oxytetracycline (terramycin) resistance in the late 1990s.

Eischen, F. A., Graham, R. H., & Cox, R. (2005). Regional distribution of Paenibacillus larvae subspecies larvae, the causative organism of American foulbrood, in honey bee colonies of the western United States. Journal of economic entomology, 98(4), 1087-1093.

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