Here’s what you’ll learn when reading this story:

• In 1957, a species of South African honey bees escaped an experimental apiary in Brazil, interbreeding with local European honey bees and creating Africanized honey bees, known more commonly as killer bees.
• Killer bee stings aren’t more deadly, but the species swarms in greater numbers due to African survival adaptations from threats like honey badgers, anteaters, and army ants.
• Over the next several decades, the bees have spread as far as the southern U.S. and will continue to progress northward as the climate warms.

Despite the honey bee’s central role in pollinating one-third of the food we eat in the U.S., North America doesn’t actually have native honey bee species. Instead, the typical “nice” bees we encounter on lovely walks around the neighborhood originally hailed from Europe, having been brought over to the Americas in the early 1600s. But there’s another kind of honey bee that, since the 1950s, has been slowly spreading across the western hemisphere. Known as an “Africanized bee,” these honey bees are more aggressive than their European counterparts thanks to evolutionary adaptations that emerged through interactions with army ants, honey badgers, anteaters, and, as always, humans themselves.

Through hybridization, these traits where transmitted to honey bees in the Americas. The hybrids are known colloquially as “killer bees” because they attack intruders in much larger swarms than typical European honey bees do. But the only reason they’re a threat in the Americas is because of one well-intentioned science experiment gone wrong.

In the 1950s, the Brazilian government asked geneticist Warwick Estevam Kerr to develop a bee that was capable of handling the challenging environment of the Brazilian Amazon. Knowing bees in South Africa were famous for their astounding honey production, Kerr decided to retrieve some specimens with the hopes of breeding them with the more docile native bees in Brazil. Kerr, fully knowing the aggressive nature of Africanized bees, believed that crossbreeding them with Brazilian bees would make the population less hostile, creating a hybrid that was literally the best of both worlds.

What Kerr didn’t plan on was an assistant beekeeper removing the queen excluder that kept the queen bees isolated within their respective hives. Of the 35 specimens in the experimental apiary in Rio Claro, located in São Paulo, Brazil, 26 queens escaped and mated with local European drones. The result was a bee well-suited to the South American climate that, to Kerr’s dismay, also retained its highly defensive nature.

Migrating 200 to 300 miles per year by swarming, these Africanized honey bees eventually made their way through central America and crossed over into the United States. In the summer of 1993, following the first confirmed U.S. death caused by these hybrids—a South Texas rancher died after a swarm attack—the newly-named “killer bees” continued to spread throughout the southern U.S., with at least 13 states having reported sightings or incidents. According to the Smithsonian Institute, killer bees have caused more than 1,000 deaths since first released in 1957. But their progress into more northern climates has been curtailed by the fact that they originated in tropical climates, so they lack the genetic adaptations needed to survive cold winters.

However, that could change. As the climate warms, experts warn that Africanized bees will continue moving northward. Research in 2014 concluded that within the coming decades, swarms could reach as far as southeastern Oregon and the southern Appalachian mountain range. Beekeepers can try to limit the spread of these hyper-aggressive bees through techniques like “drone-flooding,” which essentially means increasing the chance of European honey bees mating with a queen, and frequent “requeening,” which confirms that the queen is also always a European honey bee. But the Africanized honey bee is likely here to stay regardless.

So if you’re in place with a warm climate year-round, and you come across a swarm of honey bees—always tread lightly.