Spying Genetically Engineered Crops

August 28, 2003 at 10:51 am

Environmental Protection Agency scientists are exploring the use of satellites to monitor genetically engineered crops.

At ground level, genetically modified corn plants don’t look any different from conventional ones, but data suggest that satellite sensors may be able to read different spectral signatures from the two types of the crop. USDA

Most of these genetically modified (GM) plants have been endowed with the ability to produce a poison against the insect pests that eat them. Typically, this comes in the form of a gene from a bacterium that preys on the pest. One anticipated problem, however is that insects will become resistant to crops’ internalized poisons, much as they have grown immune to many conventional insecticides.

From the sky, satellite cameras pick up subtle spectral differences in plants that look almost identical at ground level. EPA scientists hope that these readings will be able to distinguish conventional and GM crops and to pick up when GM crops are under attack by pests. John A. Glaser, technical manager of EPA’s Biotechnology Research Program in Cincinnati, says he and his colleagues hope that satellites will detect any insect resistance to GM crops in time for farmers to change their pest-fighting strategy before harvest.

Currently, the use of satellite imagery to monitor GM plants is only in the “proof of concept stage,” Glaser says. However, major tests of the technology scheduled for next year will focus on corn, probably in Iowa and Pennsylvania. He described the research earlier this month in Chicago at an EPA Emerging Pollutants Workshop.

Corny story

The roughly 20 million acres of GM corn now growing on U.S. farms represent about one-quarter of the nation’s crop. The bioengineered corn carries a gene for and thus produces a toxin called Bt, which comes from the bacterium Bacillus thuringiensis.

Glaser explains that the corn can make 25 times the concentration of Bt toxin needed to kill almost every European corn borer, corn ear worm, fall army worm, and southwestern corn borer that might attack a plant. The only major larval pest that Bt corn doesn’t tackle is corn rootworm–and a GM plant able to fight that bug is only about a year away, he notes.

What makes GM crops so attractive to farmers–and EPA–is that the natural poison they make is lethal only to a very narrow range of organisms. While it may wallop corn earworm larvae, for instance, it leaves grasshoppers, aphids, cows, and people unharmed. Moreover, by incorporating the genes for such a poison directly into a vulnerable crop, farmers can confidently apply far fewer chemical pesticides, most of which can indeed harm friendly insects, wildlife, and people.

However, like conventional pesticides, even the plant-incorporated Bt toxin can lose its effectiveness over time. That’s why EPA requires growers who plant GM crops to adopt strategies designed to thwart insects’ evolving pesticide resistance. Chief among them: Farmers must reserve a certain share of their acreage for unmodified crops. For instance, Glaser observes, for every 80 acres planted with GM corn, a farmer must plant another 20 with conventional corn. Moreover, those conventional-corn plots must be sited within a half mile–and preferably within a quarter-mile–of the GM corn.

In this strategy, the conventional corn is a refuge for bugs. Because insects in the refuge aren’t heavily exposed to the crop’s pesticide, most of them don’t grow resistant to it. A certain share of these nonresistant insects breed with those that survive exposure to the Bt crop. This cross “dilutes” the Bt-resistance genes in the insect population, Glaser says, increasing the share of offspring that remain vulnerable to the GM corn’s toxin.

Seeing infrared

Although there are biochemical assays for confirming whether a corn leaf is making Bt toxin, they’re too cumbersome for a nationwide program that would evaluate whether farmers are complying with the requirement to plant refuge acres. That’s what makes satellite imaging attractive, Glaser says. Preliminary tests indicate that infrared photos can distinguish GM-corn plots from fields planted with conventional corn.

EPA has access to satellite imaging with a resolution that allows it to “see whether a person standing in a corn field has on glasses or not.” says Glaser That would be more than sufficient to tell whether conventional corn has been planted within a half-mile of GM crops. Tests next year, he says, will look for patterns of wavelengths that can reliably distinguish GM from conventional crops and filter out signal noise from other factors–such as drought, nutrient imbalances, or other stressors–that might mask the GM-conventional-crop differences.

If the test proves that satellite monitoring of refuges is feasible, Glaser says that the next step will be to evaluate whether such monitoring can also pick out a GM-corn crop that’s under attack by Bt-resistant insects. This may show up as a yellowing of unhealthy leaves or a weaker signal because insects have eaten a significant proportion of the crop. A satellite should be able to sense either of these changes, says Glaser.

“This whole remote-sensing effort is sort of an early-warning system that we’re trying to put together,” he says. It’s not designed “to be the end-all” of EPA monitoring of GM crops, just a tool. Agency officials want to be proactive in ensuring that the “valuable asset” of natural pesticides in GM crops remain effective for as long as possible.