March of the weevils: how a Mexican beetle launched a hundred-year attack on United States cotton

Natural History, Feb, 2006 by Robert W. Jones

Meanwhile, undeterred by impending defeat in the north, the boll weevil remains on the march in South America. It was apparently introduced into the northern part of the continent during the late 1940s and, in a mirror image of its U.S. invasion, has moved steadily southward. The insect has now invaded all the major cotton growing regions of Brazil and Paraguay, and has entered northern Argentina. Fortunately, this time pest managers have a much greater range of management options to choose from. Eradication programs in parts of South America are underway.

Two obvious questions arise from the story of the boll weevil's U.S. invasion: why did the insect appear when it did, and where did it come from? The answer to the first question is historical, and relatively straightforward. The arrival of the pest in the U.S. can be traced to economic changes wrought by the Civil War. The Union's naval blockade of the Confederacy prevented Confederate States from shipping cotton to European textile mills, sending cotton prices skyrocketing worldwide. Mexican farmers, who retained access to European markets, responded by planting more cotton, particularly in northern Mexico. In the years before the Civil War, U.S. cotton farms had been separated from their Mexican counterparts by the wide, cotton-free region of northeast Mexico. But shortly after the war, the northward expansion of Mexican cotton farming had narrowed that region substantially, bringing the cotton fields of the two nations within weevil-flying distance. Increased trade between the blockaded South and Mexico may have also helped the weevil cross the border.

But answering the first question only underscores the lack of an answer to the second: where did the boll weevil originate? Entomologists agreed from the very first that, because the weevil was previously unknown in the U.S. or South America, it must have come from southern Mexico, where it was first collected, or perhaps from Central America. They also concurred that the boll weevil, like many other plant-eating insects, restricts its reproduction to a narrow range of host plants. Hence cotton and its close relatives, all members of a group known as the cotton tribe within the family Malvaceae, were the only plants from which the boll weevil could have come. Entomologists and botanists all concluded that the insect's origins must be closely linked to the diversity and geographic range of the cotton tribe.

Those two clues prompted many investigators to search Mexico and Central America for the cradle of the boll weevil. At first, biologists discovered boll weevils only on wild or cultivated cottons--all species of Gossypium--so they assumed the insect was restricted to that genus. Searches for close relatives of cotton in Mexico and Central America later turned up two more genera from the cotton tribe, Thespesia and Cienfuegosia, that host boll weevils. But hosts from these two genera invariably had few weevils, which occurred only when the plants grew near fields of cultivated cotton, suggesting that neither genera was likely to have been the boll weevil's ancestral host.