Each year thousands of research hours and hundreds of thousands of research dollars are spent to battle caterpillars that love to eat cotton. The pink bollworm, saltmarsh caterpillars, and beet armyworms have rapidly chewed their way through acres of cotton, in some years totally decimating a crop. Cotton growers fight to produce a salable product using chemical sprays, natural controls, cultural practices, pheromones (insect mating hormones) and monitoring.
After years of development, a completely new kind of tool is available for Arizona growers to use in warding off the pink bollworm, one of the state’s major cotton pests. Transgenic cotton, a genetically engineered cotton, carries its own insecticide within the plant tissues (See How Bt Cotton was developed).
To study the effects of transgenic cotton in the field, a unique team of university, USDA and seed industry scientists have come together with growers and consultants from cotton commodity groups and other organizations in Arizona. Cooperating institutions include The University of Arizona, Arizona Cotton Growers Association, Arizona Cotton Research and Protection Council, Arizona Department of Agriculture, Cotton Incorporated, USDA/ARS (Agricultural Research Service) Western Cotton Research Laboratory, and USDA/APHIS.
These groups are studying the effectiveness of Bt cotton against the pink bollworm and other insects, its yield and cultural characteristics, insect resistance management, and how the cotton works in combination with other control measures. They are also working with local growers to promote area-wide cooperation in fighting cotton insects. A community-wide approach results in greater insect control, according to previous studies in Arizona.
UA entomologist Peter Ellsworth points out that Bt is a larvicide only. “The toxin must be ingested to kill the insect,” he says. “Therefore it does not affect adult insects.” He also notes that the bacterium kills only lepidopteran larvae (caterpillars); it is not effective on other important cotton pests such as lygus bugs, whiteflies (see IGR story in this issue) or thrips.
Ellsworth stresses that, because the insect must feed to be killed, monitoring strategies must be adjusted. “There should be less reliance on adult monitoring, damage symptoms and detection of small larvae for triggering supplemental insecticide sprays,” Ellsworth says. “Counts in Bt cotton should consider only those bolls which harbor live, large larvae.” Smaller larvae are often present in Bt cotton but rarely survive to adulthood.
Reducing or even eliminating pesticide spraying in transgenic fields will allow beneficial insects to survive where previously they were killed along with the pink bollworm when insecticides were sprayed. The helpful insects prey on cotton pests and can decrease their populations below damaging levels.
UA cotton agronomist Jeff Silvertooth has monitored the growth characteristics and yield of several Bt cotton lines at the Maricopa Agricultural Center and several other locations for the last three years. He has taken side-by-side measurements and comparisons of Bt and non-Bt cotton plants, and believes them to be equal agronomically.
“It is important to remember that the new Bt lines are very similar to companion non-Bt lines,” Silvertooth says. Other than a slow start early in the season for some of the new varieties, the Bt varieties Silvertooth has observed appear to do just as well overall in growth and yield as non-Bt cotton. “I see that as a general line or variety characteristic, not a Bt connection,” he explains.
In comparing yields of Bt cotton with those of non-Bt cotton, Silvertooth has found promising results. “Bt cottons are yielding as well as non-Bt lines in Arizona, and in many cases they’re performing better than their non-Bt counterparts,” he says, “presumably because they are suppressing the lepidopteran pests and reducing damage to the crop.”
Silvertooth advocates a conservative approach to planting the new varieties. “When first planting Bt varieties I would go to the better fields with the Bt cotton, and leave the tougher spots of the farm for varieties the growers are more familiar with,” he says. “There’s some merit to taking time to get used to these lines, just as with any new variety.”
Approximately 70,000 acres were planted to Bt cotton in the state of Arizona in 1996. According to Silvertooth, he would not be at all surprised to see an increase to 150,000 acres or more in 1997, with as much as 50% of the state’s cotton acreage planted to Bt varieties.
About ten years ago, Monsanto scientists inserted a toxin gene from the bacterium called Bt (which is the nickname for Bacillus thuringiensis) into cotton plants to create a caterpillar-resistant variety. The gene is DNA that carries the instructions for producing a toxic protein. The toxin kills caterpillars by paralyzing their guts when they eat it. Plants with the Bt toxin gene produce their own toxin and thus can kill caterpillars throughout the season without being sprayed with insecticide. Because the toxin is lethal to caterpillars, but harmless to other organisms, it is safe for the public and the environment.
Monsanto registered their Bt gene technology under the trademark Bollgard®, and authorized selected seed companies to develop cotton varieties carrying the patented gene. In 1995 the EPA granted final clearance for the first Bt-carrying cotton variety, called NuCOTN, released by the Delta and Pine Land Company. Other seed companies such as Stoneville and Hartz are incorporating the patented technology into their cotton lines as well.
After the seed technology was developed, and enough seed was available, tests began in Arizona at The University of Arizona Maricopa Agricultural Center and other locations to determine the field performance of the new varieties. By 1996, these first Bt cotton varieties became commercially available.
Managing Insect Resistance to Bt Cotton
While recognizing that transgenic plants are one of the biggest developments in insect pest management in the past three decades, some scientists worry that it might turn out to be a short-lived success. Tim Dennehy, group leader of the UA Extension Arthropod Resistance Management Laboratory, emphasizes that putting this environmentally safe insecticide into cotton plants results in the equivalent of spraying fields with the toxin every day of the season.
“I put a lot of faith in history, and since the late 1940s almost every pesticide that has been used many times per season was rendered ineffective by resistance in a matter of a few years,” Dennehy says.
USDA scientist Alan Bartlett shares this concern and has conducted studies that confirmed the expectation that the pink bollworm has the potential to become resistant to the newly developed transgenic varieties. Bartlett and Dennehy are collaborating with the Arizona Cotton Growers Association to monitor the susceptibility to Bt of pink bollworm collected from the major cotton production areas throughout Arizona.
Producers of Bt cotton have devised resistance management strategies for Bt cotton. They require that growers plant either a 4% or 20% refuge of non-Bt cotton within their Bt cotton acreage. In these refuges pink bollworms can grow without being selected for resistance by Bt.
“This is not a bad idea, Dennehy states, “however, there may be better solutions for Arizona. We need to comply with the Monsanto strategy while still evaluating the merits of other strategies, such as seed mixtures, area-wide use of Bt, alternating years of Bt use, and mixtures of Bt cotton with other environmentally-safe technologies.” Such strategies, he asserts, might double or triple the effective life of this valuable new technology.
Theo Watson, a retired UA entomologist who still works with growers also agrees that there may be better ways to use Bt cotton. He has researched mixtures of non-Bt and Bt seed and found that a 90% Bt/10% non-Bt mix held up consistently to lepidopteran attacks, especially in the late season when pink bollworm pressure was high. Although Bt cotton is particularly effective against the pink bollworm, Watson found that it is also knocked down populations of the cotton leaf perforator and beet armyworm.
Bruce Tabashnik, the new head of the UA Department of Entomology, is a recognized world expert on management of Bt resistance. He agrees with the urgent need for scientific study of Bt deployment strategies.
“Pest resistance is inevitable, but good management can substantially extend Bt’s effectiveness,” Tabashnik says. “Growers, seed companies, government agencies, and university scientists must work together to carefully track pest responses to different deployment patterns of Bt cotton.
“The potential gains from this teamwork are enormous,” Tabashnik states. “What we learn in the next few years in Arizona can help to sustain the profitability not only of the currently available Bt cotton, but also other transgenic cottons and other transgenic crops that will be central to agriculture of the 21st century.”
Article Written by Susan McGinley and Joanne Littlefield,
ECAT, College of Agriculture