Public hearings were dominated by recitations of the ills often attributed to genetically modified organisms, or G.M.O.s: cancer in rats, a rise in childhood allergies, out-of-control superweeds, genetic contamination, overuse of pesticides, the disappearance of butterflies and bees.

Like some others on the nine-member Council, Greggor Ilagan was not even sure at the outset of the debate exactly what genetically modified organisms were: living things whose DNA has been altered, often with the addition of a gene from a distant species, to produce a desired trait. But he could see why almost all of his colleagues had been persuaded of the virtue of turning the island into what the bill’s proponents called a “G.M.O.-free oasis.”

“You just type ‘G.M.O.’ and everything you see is negative,” he told his staff. Opposing the ban also seemed likely to ruin anyone’s re-election prospects.

Yet doubts nagged at the councilman, who was serving his first two-year term. The island’s papaya farmers said that an engineered variety had saved their fruit from a devastating disease. A study reporting that a diet of G.M.O. corn caused tumors in rats, mentioned often by the ban’s supporters, turned out to have been thoroughly debunked.