The killer bee panic of the late 20th century began with a farming problem. In 1956, Brazilian biologist Warwick E. Kerr imported African honeybees to cross with European strains that were sluggish in tropical heat. He kept them near Rio Claro behind queen excluders, metal screens that let workers out and kept queens in. In October 1957 a visiting beekeeper, thinking the screens were slowing the workers down, took them off. Twenty-six swarms escaped, and their descendants crossed the Amazon and Central America, reaching Texas by 1990 at up to two kilometers a day.

This episode separates the horror movie from the biology. The venom of an Africanized honeybee is identical to that of a gentle European bee. The danger is behavior: a huge alarm zone, far more guards, and pursuit of up to 400 meters. It also explains how the hybrids outbred and overthrew European colonies, why a plan to dilute them at Panama could not have worked, and how the feared invader became the preferred bee of Central and South American beekeepers, prized for heavy honey yields and for fighting off the mites that are destroying European hives.

• Over 90 percent of bees at the front of the expansion carried African mitochondrial DNA, meaning an unbroken female line ran from the escaped queens to Texas.
• In a tactic called usurpation, Africanized bees slip into a weakened European hive, kill its queen, and install their own.
• The alarm pheromone smells like artificial bananas, and victims can receive ten times the stings of a European attack, enough to cause kidney failure.
• A single Africanized hive can produce up to 100 kilograms of honey in a season, and the bees bite the legs off parasitic Varroa mites.
• Cold winters have stalled the northward spread, while in Puerto Rico, where the bees arrived in 1994, a gentle Africanized strain has evolved on its own.