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Monsanto’s Bt corn was supposed to reduce pesticide use. The Environmental Protection Agency said as much when the corn, which is genetically modified to resist the crop-ravaging rootworm, debuted in 2003. Sure enough, as more farmers sowed their fields with Bt corn, fewer of them needed to spray pesticides to protect their crops. The share of U.S. corn acreage treated with insecticides fell from 25 percent in 2005 to 9 percent in 2010.

But now, Bt corn has become, basically, too successful: Rootworms are starting to develop immunity to this prevalent crop, driving farmers to return to insecticide use. The Wall Street Journal reports:

Syngenta, one of the world’s largest pesticide makers, reported that sales of its major soil insecticide for corn, which is applied at planting time, more than doubled in 2012. Chief Financial Officer John Ramsay attributed the growth to “increased grower awareness” of rootworm resistance in the U.S. Insecticide sales in the first quarter climbed 5% to $480 million.

The frustrating part is that rootworms’ resistance to the Bt corn gene was entirely predictable — so predictable that some companies seized it as a financial opportunity:

American Vanguard bought a series of insecticide companies and technologies during the past decade, betting that insecticide demand would return as Bt corn started losing its effectiveness. In the past couple of years, that wager has paid off. The Newport Beach, Calif., company reported that its soil-insecticide revenue jumped 50% in 2012, and company earnings climbed 70% as its stock price doubled. Its insecticide sales rose 41% in the first quarter to $79 million, with gains driven by corn insecticide.

Scientists say that so far, rootworms have only developed resistance to seeds engineered to include just one rootworm trait, and Monsanto says it plans to phase out that seed and replace it with a multiple-trait variety. But the EPA cautions that rootworms resistant to the first seed are more likely to develop resistance to other traits, too. And although Monsanto recommends crop rotation to “break the rootworm cycle,” historically high corn prices are driving more farmers to plant corn every year — and that has also increased the presence of other pests besides rootworm.

So let’s set aside, for the moment, the repetitious debates between pro- and anti-GMO contingents, and consider this simple fact: Bt corn’s success lasted all of seven or eight years before rootworm resistance popped up. The same cycle could easily repeat itself with other rootworm traits or with other pests altogether.

GMOs are supposed to make farmers’ volatile business a little more secure. But when their failure is so predictable that corporations like Vanguard can profitably bet on it, who’s really coming out on top?