The Africanized honey bees in the Western Hemisphere are of mixed descent from 26 Tanzanian queen bees of A. m. scutellata, accidentally released by a replacement bee-keeper in 1957 near Rio Claro, São Paulo, in the southeast of Brazil, from hives operated by biologist Warwick E. Kerr, who had interbred honey bees from Europe and southern Africa. Hives containing these particular queens were noted to be especially defensive. Kerr was attempting to breed a strain of bees that would produce more honey and be better adapted to tropical conditions (i.e., more productive) than the European subspecies of honey bee used in South America and southern North America. The hives the bees were released from had special excluder grates to prevent the larger queen bees and drones from getting out and mating with local queens and drones of European descent. However, following the accidental release, the African queens and drones mated with domesticated local non-African queens and drones, and their descendants have since spread throughout the Americas.