New Findings on Insect Resistance to Bt Cotton While Bacillus thuringiensis (Bt) toxins expressed in transgenic cotton have proven effective in controlling the pink bollworm (Pectinophora gossypiella), scientists at the University of Arizona in Tucson also raise concerns about the possible pitfalls of using Bt cotton in Nature. It's "good news, bad news," according to Bruce Tabashnik, head of the UA entomology department and the coauthor of the Nature paper. The article, titled "Development time and resistance to Bt crops," with UA assistant research scientist Yong-Biao Liu as lead author, concerns the use of plant "refuges" to help delay insect resistance to the toxin in Bt cotton. Scientists worldwide are scrambling to learn how to use this powerful technology most effectively. To control pink bollworm and certain other cotton pests, Bt cotton contains a gene transferred from the bacterium Bt (Bacillus thuringiensis) that lets plants produce a safe, natural insecticide. When certain moth larvae feed on these plants, they die. Because the toxin is lethal to caterpillars, but harmless to most other organisms, it is much safer for humans and the environment than traditional insecticides. "Bt cotton is working phenomenally well in Arizona," says coauthor Tim Dennehy, leader of the Extension Arthropod Resistance Management Laboratory at the UA. The problem is that caterpillars might quickly evolve resistance to the toxin in Bt cotton, thus negating its effectiveness. To delay pest adaptation to Bt cotton, "refuges" of ordinary non-Bt cotton are grown to yield susceptible moths to mate with resistant moths and dilute their resistance genes. The idea is that the bollworms in these refuges do not develop resistance to the Bt toxin because there is no selection for resistance, and the toxin-resistant bollworms which do develop in the Bt cotton will be most likely to mate with a susceptible bollworm from the refuge rather than with another rare resistant individual. "The bad news is that scientists from the UA and the United States Department of Agriculture readily selected a strain of pink bollworm in the laboratory that survives on Bt cotton," Tabashnik said. "Further, resistant caterpillars develop more slowly on Bt cotton than susceptible caterpillars do on non-Bt cotton. This developmental delay could quicken the evolution of resistance to Bt by increasing the chances of resistant moths mating with one another, rather than with susceptible moths from refuges. However, in the field, higher mortality associated with slower growth of resistant caterpillars could diminish such potential negative effects." The good news is that offspring produced by matings of resistant and susceptible parents are killed by Bt cotton. Also positive is the finding that even the resistant strain suffered 63% mortality on Bt cotton. "Refuges remain vital. Our findings will help refine strategies for keeping this valuable technology effective," Tabashnik said. This research was supported by the University of Arizona, USDA, Arizona Cotton Growers Association, Cotton Incorporated, The Cotton Foundation and Monsanto. The US National Cotton Council (NCC) said it remains convinced that current resistance management strategies for pink bollworm - including the use of refugia - are appropriate and reaffirmed its commitment to preserving the effectiveness of Bt cotton. Frank Carter, NCC's manager, pest management, said there has been no demonstrated resistance to Bt in pink bollworm populations in the field. He said cotton growers recognize the threat of resistance and currently set aside a portion of their crop in non-genetically modified varieties (refugia) to ensure that the Bt product maintains its effectiveness. This resistance management strategy is based on a collaboration among scientists from leading universities and the USDA's Agricultural Research Service, and has been approved by the US Environmental Protection Agency. "This (Arizona) laboratory study infers that delayed development would somehow interrupt the randomness of mating of resistant moths surviving the Bt cotton with the susceptible ones in the non-Bt cotton refugia," Carter said. "While NCC supports research that contributes to our knowledge base on pink bollworm, these findings must be studied more thoroughly under field conditions. It is premature to conclude that deployment of refuges is not an effective resistance management strategy." Carter, an entomologist, said, "in a field situation, the cotton plants, along with tens of thousands of insects and plants are on different development schedules. Pink bollworms moths emerge, mate and reproduce every day from early spring to mid-summer, consequently, there will be ready supply of susceptible moths present if any resistant moths emerge. This supports the position that the refuge system is a valid strategy for resistance management in pink bollworm. "This information (Arizona study) is encouraging in that this is further evidence that Bt resistance is recessively inherited and secondly, that the refuge strategy will work by countering delayed development of resistant moths by providing a continuous daily supply of moths to mate with those surviving the Bt cotton. This is good news." John Foster, a professor of entomology at the University of Nebraska and a member of a national committee working to develop refuge management policy, said. "There is nothing new here. Entomologists working on insect resistance management issues have long known about developmental asynchrony and have considered it in proposing defences against resistance. Asynchrony is in every entomology textbook. I have no problem with the research being done, but it's a little surprising it is being released at this time and with the implication that it has uncovered something new and significant in regards to resistance management. "I also take exception to the statement that these laboratory findings contradict an important assumption of the refuge strategy. Nothing is contradicted until it is proven in the field. A laboratory experiment, where populations are artificially controlled does not replicate a field environment. "In the field, all insects do not emerge at the same time and they don't all develop at the same rate. There is considerable overlap in generations. Therefore, there will be populations of susceptible insects to mate with any resistant insects. I think this study has very little relevance to current strategies for insect resistance management." John Benedict, professor emeritus at Texas A&M University argued "I believe the laboratory findings are insignificant relative to the strength or value of the refuge strategy. Furthermore, the findings are insignificant relative to the societal value of Bt technology. When you consider the perspective of sustainability, integrated pest management, reduction in insecticide applications and overall societal value, Bt technology gets all A's. That should not be lost in the narrow focus on resistance management." Michael Phillips, the Biotechnology Industry Organization's executive director of food & agriculture expressed surprise that the researchers claim their narrowly focused laboratory study of larval insect development has relevance to the effectiveness of refugia strategies for insect resistance management. "While we appreciate and encourage ongoing research on techniques that farmers are successfully using to grow crops in a more sustainable manner, we're concerned that these preliminary studies are being misrepresented or exaggerated in ways that are misleading to farmers and the public," Phillips said. "Too often scientific journals publish findings that are preliminary or so narrow in scope that they lead people to jump to the conclusion that they offer substantial new scientific information," Phillips said. "Unfortunately, that first shot is heard 'round the world. Yet when further research puts the findings in context, rarely is that fuller perspective reported," Phillips said. According to Phillips, "This laboratory study does not represent anything new to entomologists who commonly observe developmental asynchrony in nature. In fact, the current resistance management practices for Bt cotton were formulated assuming that developmental asynchrony would occur," Phillips said. "Because most insects in nature emerge and develop at different times, there is a significant overlap or mixing of generations. Consequently, populations of susceptible insects will be present to mate with any resistant insects," Phillips said. For further information, contact Bruce Tabashnik, Department of Entomology, University of Arizona, Tucson, AZ 85721, USA. Fax: +1 (520) 621 1150 Tel: + 1 (520) 621 1141 Email [EMAIL PROTECTED] December 1999 -- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] Archive: http://www.mail-archive.com/[email protected]/
