Experts rear worms to extend cotton’s resistance
Posted on Sunday, September 28, 2008
FAYETTEVILLE — Insectresistant cotton has taken Arkansas by storm since 1996, when genetically engineered crops were first introduced commercially.
For the past two years, a small group of entomologists at the University of Arkansas System’s Division of Agriculture has been working to delay insects’ development of resistance to the new cotton varieties.
“It’s a whole different world of insect management,” said entomologist Randy Luttrell, referring to the shift by cotton farmers that began more than a decade ago — away from relying solely on insecticides and toward planting cotton varieties that offer built-in insect protection.
Known as Bt cotton, these varieties contain one or more foreign genes derived from the soil-dwelling bacterium Bacillus thuringiensis; hence the abbreviation Bt. The Bt genes cause cotton plants to produce toxins similar to those from crystal proteins in the bacteria, often referred to as Cry-proteins, that kill some of the most injurious caterpillar pests. Targets include the larvae of bollworms — also known as corn ear worms — and tobacco budworms.
When susceptible insects eat the leaves or bolls of Bt cotton, their own digestive enzymes activate the toxic form of the Bt protein. These proteins bind to specific receptors on the insect’s intestinal walls, causing them to rupture. Susceptible insects stop feeding within a few hours after taking their first bite and usually die within two or three days.
Since 2006, Luttrell and fellow entomologist Ibrahim Ali have received numerous sample colonies of live bollworms from cooperating researchers working in 10 states across the Cotton Belt.
“People send us insects from across the United States, and we sample Arkansas extensively,” Luttrell said.
Ali and a small team of technicians rear the sample bollworms — from the egg through the caterpillar, pupa and moth stages — and subsequent generations in a dedicated laboratory building at the Agricultural Experiment Station in Fayetteville. They are working on contract for the U. S. Environmental Protection Agency, the U. S. Department of Agriculture and the major biotechnology firms.
The insects, which are exposed to Bt toxins in an artificial diet, are monitored for the development of any resistance to Bt proteins.
POPULAR SOLUTIONS Insect-resistant cotton technology was first available to farmers 12 years ago, when the Monsanto Co. began selling a cotton variety known as Bollgard that produces a protein called Cry 1 Ac. In an effort to preserve the technology by delaying resistance, farmers who use Bollgard must provide “structured refuges” for susceptible pests by planting a certain portion of their cotton fields in non-Bt varieties. Federal regulations currently require non-Bt cotton refuges of 5 percent for single gene Bt cottons like Bollgard, if not sprayed with insecticides, or 20 percent if sprayed.
The theory is that the rare bollworm moth with resistance that survives on Bt cotton will be likely to mate with a susceptible insect that emerges from a refuge. The hybrid offspring of such a mating generally are susceptible to the toxin.
In 2003, Monsanto introduced a second-generation Bt cotton variety. Known as Bollgard II, it’s a so-called stacked-gene variety because it incorporates more than one foreign gene. Bollgard II cotton produces two Bt proteins: Cry 1 Ac and Cry 2 Ab 2.
In 2004, Dow AgroSciences introduced WideStrike, a cotton variety that produces both Cry 1 Ac and Cry 1 F.
Special non-Bt cotton refuges are not required for the stackedgene varieties, Luttrell said.
“The idea is, because it has such high killing power, the amount of susceptible gene-pool that needs to be there could be much less than what it could be for the other [earlier type of Bt cotton ],” he said.
Syngenta is developing yet another Bt cotton variety. Known as VIP or “vegetative insecticidal protein” cotton, it will produce a noncrystal form of Bt protein.
This year, an estimated 30 percent of Arkansas’ 600, 000-acre cotton crop was planted using only insect-resistant varieties, according to the U. S. Department of Agriculture.
Another 64 percent of the crop was planted using stackedgene varieties that offer insect resistance and / or herbicide tolerance, and 4 percent of the crop was planted in herbicide-tolerant only varieties.
Herbicide-tolerant cotton varieties allow farmers to spray a certain herbicide without worrying that it will kill their crops along with the weeds. Monsanto offers Roundup Ready and Flex cotton varieties, which are tolerant of the herbicide glyphosate, also known as Roundup. Bayer CropScience offers LibertyLink cotton, which tolerates the herbicide Liberty.
Randy Veach of Manila, who grows cotton in Mississippi County, said he uses Bt cotton almost exclusively. He selects varieties, based upon a field’s soil type and whether it’s irrigated, that will maximize yield per acre.
The shift to Bt cotton was made about the same time as boll weevil eradication efforts began in Arkansas, Veach said.
“The boll weevil program pretty well dictated to us that we had to start raising Bt cotton, because we were going to be spraying a lot more for bollworms if we didn’t,” he said. One result of these combined scientific advances has been a significant improvement in cotton yields. Arkansas’ statewide average cotton yield increased from 793 pounds per acre in 1996 to 1, 071 pounds per acre in 2007. A record yield of 1, 125 pounds per acre is forecast for this year. The increased yield does come at a cost, Veach said. So-called technology fees for insect-resistant and herbicidetolerant cottonseeds cost $ 60 to $ 100 per acre, he said. The fees are added to the seed price to help seed producers cover their research and development costs.
ACADEMIC DISAGREEMENT Despite concerns that bollworms are likely to develop resistance to Bt proteins, no firm evidence has been found to date, Luttrell said late last month. He differs on this point with Bruce Tabashnik, the head of the entomology department at the University of Arizona. In February, Tabashnik wrote in the journal Nature Biotechnology that a strain of bollworms had evolved that was resistant to Bt toxin. He based that article on data collected by Luttrell and Ali between 2003 and 2006.
But while Luttrell’s data does fit Tabashnik’s model, Luttrell said he interprets it differently.
As early as 1992, bollworms in the field could be found with the genetic capacity to develop Bt resistance, Luttrell said. However, he has yet to see evidence of sustained resistance in fieldcollected insects.
Researchers also have detected a “fitness cost” associated with Bt resistance, Luttrell said.
“If [insects are ] carrying any sort of resistance, they tend to not be as fit. They don’t survive to produce insects as well as those that are not carrying those traits,” Luttrell said.
The debate over Bt resistance is likely to continue for a year or two, he said.
The EPA registration of Bt cotton varieties is contingent on being able to manage and contain such resistance, Luttrell said.
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