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The Africanized honey bees in the Western Hemisphere are descended from hives operated by biologist Warwick E. Kerr, who had interbred honey bees from Europe and southern Africa. Kerr was attempting to breed a strain of bees that would produce more honey and be better adapted to tropical conditions (i. e. , be more productive) than the European strain of honey bee currently in use throughout North, Central and South America. The hives containing this particular Africanized subspecies were housed at an apiary near Rio Claro, São Paulo, in the southeast of Brazil, and were noted to be especially defensive. These hives had been fitted with special excluder screens (called queen excluders) to prevent the larger queen bees and drones from getting out and mating with the local population of European bees. According to Kerr, in October 1957 a visiting beekeeper, noticing that the queen excluders were interfering with the worker bees' movement, removed them, resulting in the accidental release of 26 Tanganyikan swarms of A. m. scutellata. Following this accidental release, the Africanized swarms spread out and cross-bred with local European colonies.