COLLEGE STATION — New versions of genetically engineered, insect- resistant Bt cotton and other crops will be in use within five years. And even though the original versions have just been planted, new strains will be necessary, according to an entomologist with the Texas Agricultural Experiment Station.
That’s because science may be able to control insect pests, but nature is equally adept in reacting to the wonders of science, said Dr. John Benedict of the Texas A&M Agricultural Research and Extension Center in Corpus Christi.
Benedict, working with private industry, helped evaluate the genetically engineered Bt cottons containing genes that produce an insecticidal protein from the bacterium Bacillus thuringensis. The cotton strains were released for 1996 plantings.
The protein is toxic to many caterpillar pests when ingested. It can help farmers reduce use of insecticides that also kill beneficial insects that feed on cotton- munching whiteflies and aphids. However, there is a catch.
“Even though Bt cotton is resistant to tobacco budworms and bollworms, there’s some concern that it may be too good at resisting them,” Benedict said. “The Bt gene is expressed in every cell of the plant, and because it’s toxic to the worms, it puts pressure on the worm populations to become resistant to the toxin.
“In other words, the ones that survive are likely to be the ones that are resistant, and the more widely we use these Bt plants, the more likely we are to select for the most resistant worms.”
That’s why experiment station researchers and private industry are already working on the successors to Bt cotton. Despite the best efforts of scientists, some insects can or do develop their own resistance to plants that are toxic to their kin, meaning a cycle of continual improvement may be needed to keep crops safe from pests. With conventional synthetic chemical insecticides, a cycle of about six to 15 years is normal for its effective life span, and then pests begin to develop some resistance, Benedict said.
Some computer simulation models predict Bt cotton, released in 1996, will remain effective for only two to five years, but, Benedict said, “there are ways to keep a specific insecticide in the market longer than the standard period, particularly one as environmentally friendly as Bt cotton.”
Benedict said private-industry scientists are “pyramiding” genes into cotton — that is, inserting into a plant additional genes that produce proteins capable of attacking specific sites on an insect’s gut.
The method is the same as used originally to insert the Bt gene into cotton, but with more genes. Once new genes have been successfully expressed in a plant, that plant can simply be crossed with existing varieties to produce the new resistant variety with multiple genes.
The researchers expect to have, within five years, commercially available Bt cotton containing two genes contributing to resistance, Benedict said.
Station researchers are working in concert on the strategy with the Monsanto Corp., which is testing several potential proteins for pyramiding. Benedict said Bt cotton eventually will contain three different genes producing proteins that help resist insects.
“You can get a broader spectrum of insect control through pyramiding,” Benedict said. “Monsanto is hoping to control beet armyworm and fall armyworm, which are not impacted much by current Bt cottons. Also, various proteins attack different targets within insects, so it’s more difficult for pests to develop resistance to the pyramided cottons than to single toxins.
“It’s a rare event to find an insect that can cope with one protein, and even rarer to find one that can cope with both.” The proteins have two other key advantages over traditional pesticides, Benedict said.
“Most of the new ones are not toxic to man and animals except the target pest, and they have no effect on the environment,” he said. Benedict added that other strategies may aid in the effectiveness of new, resistant crops.
One strategy is to leave limited amounts of acreage planted in non- resistant cotton so small populations of susceptible pests survive to mate with any potentially resistant pests, diluting the potential for resistant strains.
“Realistically, we need to understand that we need new strategies to keep resistance in pests to a minimum,” Benedict said. “Monsanto has actually developed a resistance management strategy, and I know of no other company which has done that to get its insecticide registered with the Environmental Protection Agency.”
Such planning is needed to help maintain the natural diversity among both plants and the pests that feed on them, as well as helping agricultural producers cut conventional pesticide use.
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