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Genetically engineered (or GMO) corn that was developed to kill insect pests has been causing resistance in some of those very insects for several years. But the issue has not received as much attention as it should. A new research paper in the prestigious journal, Proceedings of the National Academy of Sciences (PNAS), may change this by providing additional cause for concern.

Rootworms in some areas are now resistant to two of the three available Bacillus thuringiensis (Bt) toxins that previously controlled them. And because only one or two of these toxins are used per corn variety, a number of fields in the corn belt are now sustaining severe damage.

Bt was hailed early on for reducing the need for chemical insecticides. But resistant rootworms may be starting a reversal of this trend, noted by University of Illinois Entomologist Michael Gray last spring. Some farmers are reverting to the use of harmful soil-applied insecticides to kill the beetles’s larvae.

Two Strikes, and Other Bad News

The new paper is also notable for several other reasons.

It found that the same rootworms that are resistant to Monsanto’s Bt toxin are also resistant to one of the two other rootworm Bt toxins, available from Syngenta. This so-called cross-resistance means that there is one less Bt option for controlling rootworms than was thought to be available, leaving only one in some parts of the corn-belt.

This means big problems for the industry plan for stemming resistance to Bt in this major pest. Their approach was  to throw several Bts at the insect at the same time, because doing so makes it less likely for resistance to develop. This strategy, called “pyramiding,” has backfired because Monsanto’s and Syngenta’s varieties were, in effect, too similar. No pyramid, no resistance strategy!

Making the whole thing worse is the fact that the rootworm Bt corn was never as effective as the previous Bt toxins that control stem borers. This greatly reduces the effectiveness of the other main, EPA mandated, strategy for controlling resistance-requiring the planting of a “refuge” area of non-Bt corn. The refuge requirement was set at 20 percent of the crop when approved in 2003, and recently lowered to only 5 percent when pyramided varieties are used. The refuge corn provides non-resistant mates for (previously) rare resistant rootworms, thus diluting the effect of resistance genes to manageable levels. But because rootworm Bt toxin is less lethal, more rootworms survive, eventually overwhelming the refuge strategy. Rootworms can develop resistance to Bts very quickly-and did so within about four years after introduction-when the refuge area was 20 percent.

This means that the remaining effective Bt toxin will probably be quite vulnerable to resistance.

The PNAS paper also shows that resistance to Bt has likely been spreading since it was first discovered in 2009, and that the level of resistance seems to be increasing.