Africanized honeybees—the term entomologists prefer rather than killer bees—are a hybrid bee originating in Brazil, where African honeybees were imported in 1956. The breeders, hoping to produce a bee better suited to producing more honey in the tropics, instead found that African bees soon hybridized with and mostly displaced the familiar European honeybees. Although they produce more honey, Africanized honeybees (Apis mellifera scutellata) are more dangerous than European bees because they attack intruders in greater numbers. Since their introduction, they have been responsible for approximately 1,000 human deaths.

In addition to such safety issues, concern is growing regarding the effect of possible hybridization on the U.S. beekeeping industry. In October 1990, the bees crossed the Mexican border into the United States; in 1996, these bees were found in parts of Texas, Arizona, New Mexico, Nevada, and California; by 2009, they had crept up even further into those states and added southern Oklahoma, Louisiana, and Florida—even Puerto Rico and the U.S. Virgin Islands—to the list. Their migration northward has slowed—probably because they are a tropical insect and cannot live in colder climates.