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

So cotton remained the prime suspect. Then, in the 1960s, Paul A. Fryxell, a botanist who specialized in the Malvaceae family at the USDA Cotton Laboratory in College Station, Texas, realized that a relatively obscure but diverse genus of small, tropical trees by the name of Hampea had been assigned to the wrong plant family. Other botanists had classified Hampea solely on the basis of male specimens, which lack the distinctive tube of fused stamens that surrounds the female flower parts in members of the Malvaceae. Fryxell had both male and female specimens and easily recognized that Hampea belonged in the Malvaceae. Even more, it was a member of the cotton tribe.

Taxonomy is often thought of as a descriptive science with a lot of name shuffling that yields no testable hypotheses. But Fryxell's seemingly minor taxonomic change turned out to be the most important clue in the search for the boll weevil's origins. For if Hampea is closely related to cotton, might some species of Hampea also have boll weevils?

The question was answered in 1966, when Fryxell and his colleague, the late Maurice J. Lukefahr, discovered boll weevils on a common, but at the time under-scribed species of Hampea growing far from commercial cotton farms in the Gulf Coast region of Veracruz, Mexico. On the basis of the large, apparently well-established populations of boll weevils they discovered, Fryxell and Lukefahr concluded that Hampea, not cotton, is probably the boll weevil's ancestral host. Fryxell named the species Hampea nutricia, for its role in providing nutrition and shelter to the boll weevil and another cotton pest, the cotton leafworm.

Evidence gathered in subsequent years has bolstered their conclusion. In 1979 Horace R. Burke and James R. Cate, both entomologists at Texas A&M University in College Station, discovered a previously unknown weevil species (Anthonomus hunteri), living exclusively on another species of Hampea in the Yucatan Peninsula. The discovery was important because the boll weevil had always been something of an orphan within Anthonomus, its megadiverse genus. More than 300 species of Anthonomus are recorded in Central and North America alone, yet no close relative of the boll weevil had ever been found.

In the past six years Burke and I have filled in more of the boll weevil's family tree. We have described three more species of Anthonomus, closely related to the boll weevil, which also live on various species of Hampea in southern Mexico and Central America. The fact that the boll weevil's closest relatives are restricted to Hampea, whereas the boll weevil alone lives on cotton, suggests that Hampea and its associated weevils have had more time than cotton and the boll weevil to co-evolve. Hampea is thus a more likely original host than cotton.

If the boll weevil first evolved on Hampea, when did it shift to cotton? The question remains an open one, but it is intriguing to note that Mexico's Gulf Coast region along the border between the states of Tabasco and Veracruz is most likely where cotton was first domesticated in the Americas. That region corresponds precisely with the distribution of H. nutricia. The plant is a vigorous colonizer of disturbed soils, and so it is common near agricultural fields. The boll weevils that feed on it almost certainly had intimate and sustained contact with cotton plants cultivated by indigenous farmers. Boll weevils would thereby have had plenty of chances to adapt to the newly domesticated and increasingly plentiful crop.