December 16, 2003
by Dennis T. Avery
The superbugs aren’t showing up. In a major disappointment for environmental activists, insects are not building up resistance to the genetically-engineered Bt corn and cotton that have been planted on millions of acres around the world since 1995.
“If I’d gotten up seven years ago and said that there would be no evidence of increased Bt resistance after Bt crops had been planted on [150 million acres], I would have been hooted off the stage,” says the University of Arizona’s Bruce Tabashnik, an entomologist who’s just led a survey of Bt resistance.
Dr. Kongming Wu of the Chinese Academy of Agriculture Sciences says that cotton bollworms developed measurable resistance to Bt in the laboratory. However, field monitoring in the Yellow River Valley over the past five growing seasons has found no discernibly increased resistance in the wild bollworms.
Tabashnik says, “More than 500 species of insect have evolved resistance to one or more conventional insecticides. So far, the track record of Bt is better.”
To date, the diamondback moth is the only pest to evolve Bt resistance – and that resistance developed from the use of “live” Bt sprays by organic farmers over the past 40 years. (Bt is a natural poison, found in a common soil bacterium.)
It wasn’t supposed to be this way. Dr. Michael Altieri of the University of California/Berkeley wrote in a 2001 critique, “ecological theory predicts that as long as transgenic crops follow closely the pesticide paradigm prevalent in modern agriculture, such biotechnological products will do nothing but reinforce the pesticide treadmill in agroecosystems.” Altieri specifically predicted the “rapid evolution of resistance to such insect pests as moths and butterflies becoming immune to Bt.”
Greenpeace in 1999 sued the Environmental Protection Agency, demanding that all Bt crop registrations be canceled to protect organic farms from the Bt resistance they claimed would quickly develop with widespread plantings of Bt biotech crops.
One reason for the lack of resistance buildup is that the Bt plants deliver their natural toxin directly into the pests’ digestive systems, while pesticide sprays may or may not deliver a toxic dose to the pests’ external parts. Secondly, the Bt corn can make and deliver up to 25 times the concentration of toxin needed to kill almost any chewing insect. This is permitted because Bt toxin is lethal only to a very narrow range of organisms, walloping the bollworm and corn borer while leaving grasshoppers, bees, cows, and people unharmed.
Thirdly, government regulations require Bt-using farmers to plant refuges. Corn growers, for example, must plant 20 percent of their corn acreage in non-Bt varieties. The Bt resistance is a recessive trait, and any bugs with resistance (or their offspring) have a possibility of mating with bugs from the refuge, preventing any immunity from being passed on. Graham Head, who directs insect resistance management for Monsanto, says “The use of refuges to manage resistance that tends to be recessive and have fitness costs [to the attacking insect] is a highly effective means of delaying resistance.”
This doesn’t mean that Bt resistance will never be a problem. Insects are enormously adaptable. What it does mean is that we’re getting more time to develop our next line of pest defense than we expected to get, and producing higher-yield crops in the meantime. (Bt corn yields in the tropical pest-ridden Philippines are 60 percent higher than conventional yields.)
The next line of defense? Well, Bt is not just one toxin, but a dozen or so different varieties. Researchers are working to “stack” two or three different Bt toxins in each plant so that, instead of building resistance to one toxin, the bug would have to become resistant to three toxins simultaneously – which is vastly less likely.
The activist Union of Concerned Scientists is, naturally, also opposed to the stacking strategy; they say it would double the likelihood of environmental accidents. (That seems highly improbable, but the UCS would apparently like humanity to abdicate in favor of the borers and bollworms.)
Dennis T. Avery is based in Churchville, VA, and is director of the Hudson Institute's Center for Global Food Issues.
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