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Bugs Bite Back
August 16, 1999
Courtesy Monsanto

Another controversy has cropped up over alleged problems posed by plants genetically engineered to defend themselves against pests. Researchers in Arizona report that insects can quickly become resistant to such plants. In particular, the study showed that a common pest, named the pink bollworm, quickly adapted to a crop of so-called insect-resistant Bt cotton.

"We provided the first evidence of a strain of an insect that’s capable of surviving on a Bt crop that’s grown commercially in the United States," explains Bruce Tabashnik, one of the authors of the Arizona study. "I think our results are one step in a sequence of information showing it’s not theoretical. It’s a very real possibility."

image: Nebraska Game and Parks Commission

This study comes on the heels of another discouraging report about insect-resistant crops. A few months ago, scientists at Cornell University reported that pollen from Bt corn kills more than just pests—it’s also toxic to Monarch butterflies, which raises further questions as to whether the pollen might be harmful to a whole slew of other insects and animals. The Cornell finding, along with the Arizona study (which was reported in the August 5 issue of the British journal Nature) places manufacturer Monsanto in the position of having to defend the benefits of its insect-resistant Bt products.

Good Intentions

image: Monsanto

Researchers have genetically engineered Bt plants to express a gene for the bacterium, Bacillus thuringiensis. The gene instructs the plants to produce a toxin that is harmless to humans, but deadly to the pests that feed on the plants. Before the introduction of Bt plants, farmers sprayed their crops with millions of gallons of pesticide. Bt plants produce the toxin inside the plant itself. Researchers originally believed that Bt plants would offer several compelling advantages over unmodified crops treated with spray-on pesticides.

  • More control: Whether intentional or not, bugs can hide from pesticide sprays; they cannot hide from Bt.

  • Less damaging to the environment: The excess from millions of gallons of applied pesticides can run off into the soil and water supply.

  • Less expensive: Eliminates the costs of labor and chemicals.

Researchers acknowledge that pests will, over time, develop resistance to toxins. However, they disagree on how long it will take for a species to become resistant.

No Safe Refuge?

Tabashnik and his colleagues called into question the effectiveness of the "refuge strategy" for delaying resistance to the Bt toxin. The idea behind the strategy is to plant sections of unmodified crop adjacent to the Bt crop; toxin-resistant pests will mate with the non-resistant forms that were raised on the unmodified crops. Assuming that resistance is a genetically recessive trait, the offspring from the interbreeding will have a higher probability of being non-resistant; thus, they will be susceptible to the toxin produced by the Bt plants.

This strategy requires a second critical assumption—namely, that random mating occurs between the susceptible and resistant strains. While other research groups have investigated the degree of physical proximity required for random mixing to occur between the populations, the Arizona team also compared how long it took for each strain to reach reproductive maturity.

"We found something that may cause a problem and interfere with random mating," explains Tabashnik. "Namely, the resistant insects develop on Bt cotton a little slower than the susceptible insects on non-Bt cotton. That means that the timing could be off, which would reduce the tendency for matings between the two kinds of insects."

Industrial Strength Response

One criticism of the Arizona finding is that the study was carried out in a greenhouse and not under typical field conditions. "Laboratory data does not or rarely reflects what happens in a field situation," says John Foster, an entomologist at the University of Nebraska. "Until the first resistance occurs in the field on a Bt plant, we’re not going to be able to do conclusive studies." Industry organizations such as the National Cotton Council and the Biotechnology Industry Organization have issued statements defending the refuge strategy and the benefits of Bt crops.

image: Monsanto
Genetically engineered cotton has been on the market for three years and already accounts for 60 to 80 percent of Arizona’s cotton crop. Bt cotton is saving growers on the order of $40/acre/season. Considering the hundreds of thousands of acres planted, it is reasonable that both growers and industry would have a strong interest in protecting their investments. In the meantime, scientists and the federal government will be waiting and watching for any signs of resistance in the field—something nobody wants to see.

"A lot of work, effort, and energy went into developing these crops for resistance to the pest," says Foster. "If we don’t maintain that level of resistance in the crop plant, then we lose that technology. It is a very costly process."

Elsewhere on the web:

Bruce Tabashnik, Department of Entomology at the University of Arizona.

John Foster, Department of Entomology at the University of Nebraska, Lincoln

Monsanto

National Cotton Council

Biotechnology Industry Organization

EPA and USDA Workshop on Bt Crop Resistance Management

"Impacts of Adopting Genetically Engineered Crops In the US—Preliminary Results" A study from the Economic Research Service of the US Department of Agriculture



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