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ARS Home » Southeast Area » Stoneville, Mississippi » Southern Insect Management Research » Research » Publications at this Location » Publication #225609

Title: Field-evolved resistance to Bt toxins

item Abel, Craig

Submitted to: Nature Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/3/2008
Publication Date: 10/1/2008
Citation: Moar, W.J., Roush, R.T., Shelton, A.M., Abel, C.A., Ferre, J., Macintosh, S., Leonard, R. 2008. Field-evolved resistance to Bt toxins. Nature Biotechnology. 26:1072-1074.

Interpretive Summary: Field sampling and testing cotton bollworm/corn earworm for resistance to Bt cotton has occurred throughout the cotton belt before the transgenic crop was adopted in 1996 to the present day. A paper by Tabashnik et al. 2008, Nature Biotechnology 26:199-202, stated that field-evolved resistance has already occurred for this insect in the U. S. Tabashnik et al. base their conclusion on two publications by Dr. Randy Luttrell’s laboratory at the University of Arkansas in which the pest was evaluated for its resistance to the insecticidal protein, Cry1Ac, in Bt cotton before (1992) and after (2002) wide-scale adoption of the transgenic crop. Declaring the evolution of field resistance in this insect to Bt cotton based on only these two data sets is problematic for a number of reasons. One of the most critical aspects when testing populations for resistance is to use a reliable standard susceptible insect colony to accurately account for the inherent variable response of the insect to Cry1Ac. We cite the work of many others who tested this variability. This variability in susceptible cotton bollworm’s response to Cry1Ac often exceeds the change noted between 1992 and 2002 by Dr. Luttrell’s laboratory that is used by Tabashnik et al. to claim that field resistance has occurred. Also, the method in collecting and testing the insects by the Luttrell laboratory in 2002 was vastly different than in 1992, including sampling larvae directly from Bt crops which would mask any change that may or may not have been occurring in the general population. After collectively re-examining all of the available data sets, we recognize that Cry1Ac-resistant alleles may be present within H. zea populations, but there is no evidence to suggest that the frequency of these alleles has significantly increased with the use of Bt cotton. As has been done in the past, the scientific community will continue to be vigilant, monitoring the potential for this insect to evolve resistance to Bt proteins, while being equally careful not to misinterpret laboratory data, particularly where comparisons are made among very different data sets.

Technical Abstract: Transgenic cotton expressing Bacillus thuringiensis Cry1Ac (Bt cotton) has been used commercially in the United States since 1996. An article by Tabashnik et al. 2008, Nature Biotechnology 26:199-202, states that, for the first time, there is field-evolved Bt resistance in bollworm, Helicoverpa zea (Boddie) in the U. S. because of the extensive use of Bt cotton. Tabashnik et al. base their conclusions on two publications by Dr. Luttrell’s laboratory in which Cry1Ac toxicity to H. zea populations were evaluated before the introduction of Bt cotton (1992) and after (2002). The following are four concerns with the conclusions of Tabashnik et al.: 1) the definition of Bt resistance used by Tabashnik et al. is purely laboratory-based, while field efficacy and larval survival on plant tissues are the ultimate criteria for contextualizing laboratory-based estimates of resistance, and no change in Bt cotton efficacy has been documented during the past decade; 2) larval samples should not be collected from Bt crops because they will not be representative of the population as a whole, especially for highly mobile insects such as H. zea; 3) the data from Luttrell’s laboratory on which Tabashnik et al. base their conclusions use LC50’s to measure resistance, which introduces artifacts into the analysis; and 4) the baseline comparator used to assess variability in these laboratory assays is not representative of field susceptibility; when a more appropriate comparator colony is employed, the results from Luttrell’s laboratory bioassays indicate no change in susceptibility. Based upon historical and current results, it is premature to conclude that field-evolved resistance to Bt cotton has arisen in H. zea. Public scientists and the agricultural industry must continue to be vigilant and monitor for potential changes in susceptibility to Bt proteins. However, it is important to be cautious in interpreting laboratory data, particularly where comparisons are made among variable sets of data.