The Africanized honey bee is actually a variety of domestic bee derived by hybridization from African honey bees naturalized in the Western Hemisphere. Because they are very defensive and will attack perceived intruders more readily than the common European bee, they are also known by the popular name "killer bees." Brazilian scientists imported African queen bees in the 1950s in order to breed a bee for use in tropical climates. Some swarms escaped into the wild. Because they were highly adapted to tropical survival and had no natural competitors, they thrived and spread rapidly across South America, extending their range up to 500 km (300 mi) per year. By the 1980s, Africanized bees had reached Central America and soon colonized Mexico. In 1990, the first swarm was found in the United States. The bees spread from Texas to New Mexico and Arizona, then to California in 1994. The bees have reached an apparent climatic limit at their southern range in mid-Argentina, and their range is expected to also be limited to the southern and coastal states of Argentina. the United States. They have hybridized to some extent with wild populations and European bee hives. However, many of the fundamental characteristics of African bees remain, including rapid population growth, frequent swarming, minimal honey hoarding, the ability to survive on scarce reserves of pollen and nectar, and a ...