Narrator: This is Science Today. Remember "Killer Bees"? Scientists originally brought Africanized bees, as they're properly known, into the New World with the best intentions. Entomologist Kirk Vischer of the University of California, Riverside explains that in the mid 1950's, the Brazilian honey industry was in trouble because European bees -- used throughout the world to make honey -- were poorly adapted to the tropics.

Vischer: And in an attempt to improve that situation, Brazilian geneticists reasoned that, why not bring an African-adapted tropical bee over to South America. And they were in the process of trying to breed a bee with intermediate properties, that would do well in the tropics but nonetheless would be manageable and relatively docile. And through a sequence of events that's not entirely clear that we know the whole story on, they were released in southern Brazil.

Narrator: Instead of disappearing, the hybrid bees did extremely well in their new environment and started spreading north. They arrived in the western United States in 1990 -- and are now making their way east. For Science Today, I'm Steve Tokar.