The Africanized honey bee is actually a variety of honey bee derived from the hybridization of African honey bees naturalized in the Western Hemisphere. Because they are highly defensive and will attack suspected 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 to breed a bee for use in tropical climates. Some swarms have escaped into the wild. Because they were highly adapted to tropical survival and had no natural competitors, they thrived and spread rapidly throughout South America, extending their range by 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 and then to California in 1994. The bees reached an apparent climatic limit to their southern range in the middle of Argentina, and their range is predicted to be similarly limited to the southern states and coastal in the United States. They have hybridized to some extent with wild and resident hive populations of European honey bees. However, many of the key traits of African bees remain, including rapid population growth, frequent swarming, minimal honey accumulation, the ability to survive on low supplies of pollen and nectar, and a ...