field of genes

These wheat seedlings are part of a project to breed a crop that's stronger, more bountiful and more disease-resistant.
Photo © David Tenenbaum
  The dismal science of resistance
How do insects become resistant to insecticides? Courtesy of Evolution 101. If the pesticide (either synthetic or plant-made) kills almost all the bugs, only those insects with a gene that makes a chemical that somehow evades or destroys the pesticide will survive.

whestWhen these so-called "resistant" insects breed, their offspring are more likely to have the resistance gene. If the pesticide is still present, these resistant insects soon become dominant. As Dr. Darwin said, they are the fittest, and they survive. And that forces the farmer to either use more of the existing pesticide or find a new one. (This evolutionary process also occurs with antibiotic-resistant microbes. See our coverage).

Entomologists say 500 crop pests have already evolved resistance to conventional insecticides. And the buzz among people who study insect pests is that they will eventually overcome insect-killing bacteria like Bacillus thuringiensis as well. Mosquitoes have already developed resistance to Bt, and a Minnesota laboratory study showed that European corn borers developed resistance in just seven or eight generations.

"We do expect resistance to develop to Bt," conceded Monsanto scientist Rob Horsch in 1996. "That's why we began an insect resistance management analysis 10 years ago."

The theory makes sense, but with the survival of the best organic insecticide at stake, an argument has erupted over the details. Before we get to the fun stuff, let's pause to note that at least the two sides agree on the premises. Insect resistance, say Monsanto and its critics, is best deterred by:

  bullet making crops that put out a high level of insecticide (to kill all susceptible insects) and

  bullet establishing refuges where neither Bt crops nor Bt insecticides are used. The Bt-susceptible insects living in these refuges are supposed to mate with any resistant insects and dilute their resistance genes.

If resistance occurs,
organic farmers will lose one of their most valuable natural pest controls.
  Thus if any resistant bugs develop, their resistance genes would -- theoretically -- be swamped in the larger population of susceptible insects.

Got it in writing
Monsanto requires farmers to agree in writing to devote a certain acreage to refuges. Refuges sprayed with conventional insecticides must equal 20 percent of the Bt-variety acreage. Unsprayed refuges must equal 4 percent of the Bt acreage.

Still, the devil is in the details of refuge size and location. Although Monsanto is apparently the only biotechnology company that's actually requiring farmers to establish refuges, outside experts say higher set-aside percentages are warranted. The North Central Committee on the Ecology of Stalk-Boring Insects, an inter-university group concerned with European stalk borers, suggests that 20 to 25 percent of corn crops ought to be Bt-free, according to member John Wedberg, chair of the department of entomology at the University of Wisconsin-Madison. This acreage could be sprayed with non-Bt insecticides as needed.

Margaret Mellon, director of the agriculture and biotechnology program at the Union of Concerned Scientists, an interest group in Washington that wants to preserve Bt's utility for organic farmers, says, "There are good reasons to think the refuge idea could work, but it's a big leap from a plan on paper to a plan that's working in the field. That idea has not been field-validated." In light of how little is known, Mellon suggests devoting 20 to 50 percent of acreage to refuges.

The refuge design needs to be just right, she adds. "If a refuge is more than a mating flight away from the resistant insects, it might as well not be there." And the existing plans, she says, are sketchy on exactly where the refuges must be.

Despite the reassuring sound of these percentages, many scientists say they are essentially shots in the dark, and were based on the expectation that resistance genes are rare in crop pests. But a study published in 1997 shows that may be untrue. Entomologist Bruce Tabashnik of the University of Arizona and colleagues studied the diamondback moth, the first insect to develop resistance to Bt in the field (these fields did not contain transgenic crops -- they were sprayed with Bt). Tabashnik found that a single gene present in 21 percent of the moths conferred resistance to four strains of Bt (see "One Gene in Diamondback Moth" in the bibliography.)

What's the bad news?
The implication of the study, Tabashnik says, is "that insects may become resistant more readily than expected, and for two reasons. A single mutation can confer resistance to more than one Bt toxin, and the frequency of mutation is much higher than we expected."

But the results were not conclusive since the moths were lab-grown, not field-captured. Furthermore, most of the four toxins were closely related, making it more likely that a single gene could overcome them.

The seed companies have already devised an escape plan. Before resistance develops, they hope to insert genes coding for several toxins into their transgenic seeds. The premise is that it's much harder to develop resistance to several insecticides at once -- just as you're less likely to win the daily double than a single race. Says Monsanto press relations officer Dan Holman. "There are literally thousands of Bt proteins that can be developed into insecticides. Just like with any other insecticide, you are always looking for a new active ingredient."

Although all makers of Bt-bearing seeds have an incentive to preserve the extraordinarily useful proteins, organic farmers who depend on Bacillus thuringiensis as one of their best insecticides are disturbed that a giant chemical company like Monsanto is profiting from products that could destroy Bt's effectiveness. At stake in the argument is a safe, natural pesticide that's now used on dozens of crops, including 100 percent of Florida celery, 93 percent of California artichokes, and 80 percent of California eggplant.

"If resistance occurs, organic farmers and others will lose one of their most valuable natural pest controls," said Jane Rissler, senior staff scientist at the Union of Concerned Scientist. The organization has asked the EPA to set standards to slow or prevent that loss (see "Now or Never: Serious Plans to Save a Natural Pest Control" in the bibliography).

In the meantime, Tabashnik and other scientists are busy scouring Bt-seeded fields for resistant insects. In Arizona's cotton crop, Tabashnik says, such insects have not been found.

What are some other concerns about transgenic seeds?

The Why Files
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