(Newser) – In 2003, Monsanto debuted corn seeds with a gene—Bt—that generates pest-killing toxins designed to resist the ravages of rootworm. Soil insecticide use cratered as a result, with only 9% of corn acreage nationwide treated with it in 2010, down from 25% in 2005. But then things started to change: In 2011, scientists discovered rootworms with Bt resistance, and now pesticide makers say sales are once again booming. In the case of Syngenta, sales of its corn insecticide more than doubled last year; American Vanguard, which the Wall Street Journal reports has been acquiring insecticide companies based on the belief that just such resistance would come to pass, saw a 41% rise in insecticide sales in Q1.

One farmer says his yield of Monsanto corn on fields hit by rootworms was down by two-thirds last year; he'll turn to insecticides this year in hopes of avoiding a repeat. "It was a train wreck," he says. For its part, Monsanto says it is introducing a more robust version and is working on new rootworm-fighting technologies. In the meantime, an Illinois entomologist notes that about one out of every two farmers he surveyed over the winter plans to use the BT seed ... and insecticide. (Read more rootworm stories.)

