Insect resistance to Bt toxins
There is an ongoing debate going on at Nature Biotechnology about an important issue -- the emergence of resistance to Bt toxins. It starts out with the following paper:
Insect resistance to Bt crops: evidence versus theory.
Tabashnik, B.E., Gassmann, A.J., Crowder, D.W. & CarriÃ¨re, Y. Nat. Biotechnol. 26, 199 202 (2008).
Evolution of insect resistance threatens the continued success of transgenic crops producing Bacillus thuringiensis (Bt) toxins that kill pests. The approach used most widely to delay insect resistance to Bt crops is the refuge strategy, which requires refuges of host plants without Bt toxins near Bt crops to promote survival of susceptible pests. However, large-scale tests of the refuge strategy have been problematic. Analysis of more than a decade of global monitoring data reveals that the frequency of resistance alleles has increased substantially in some field populations of Helicoverpa zea, but not in five other major pests in Australia, China, Spain and the United States. The resistance of H. zea to Bt toxin Cry1Ac in transgenic cotton has not caused widespread crop failures, in part because other tactics augment control of this pest. The field outcomes documented with monitoring data are consistent with the theory underlying the refuge strategy, suggesting that refuges have
helped to delay resistance.
[The following letter in Nature Biotechnology this month disputes the researcher s conclusions. It starts as follows.]
Field-evolved resistance to Bt toxins, Willam Moar, et. al., Nature Biotechnology 26, 1072 - 1074 (2008), doi:10.1038/nbt1008-1072,
An article by Tabashnik et al.[ABSTRACT PROVIDED ABOVE by the Pundit] in your February issue states that, for the first time, the frequency of resistant alleles has increased substantially and that there is field-evolved Bacillus thuringiensis toxin (Bt) resistance in bollworm, Helicoverpa zea (Boddie), in the United States because of the extensive use of Bt cotton. Tabashnik et al. base their conclusions on two publications by Randy Luttrell's laboratory in which Cry1Ac toxicity to numerous H. zea populations was evaluated: Luttrell et al.2 (before introduction of Bt cotton) and Ali et al.3 (after introduction of Bt cotton).
We emphatically disagree with the conclusions of Tabashnik et al. that the data published in these two articles demonstrate field-evolved resistance in H. zea for four reasons: first, the definition of Bt resistance used by Tabashnik et al. is purely laboratory based, whereas 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; second, 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; third, the data from Luttrell's laboratory on which Tabashnik et al. base their conclusions have been evaluated using LC50 (median lethal dose; 50%) values to measure resistance, which introduces artifacts into the analysis; and fourth, 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, results from Luttrell's laboratory bioassays indicate no change in susceptibility. We discuss each of these aspects in turn below...(Continues at Journal)
[The original papers' authors have responded.]
Field-evolved resistance to Bt toxins, Bruce E Tabashnik, et.al., Nature Biotechnology 26, 1074 - 1076 (2008),
Bruce E Tabashnik, Aaron J Gassman, David W Crowder & Yves CarriÃ¨re reply:
We welcome the opportunity to confirm one of the main conclusions of our paper1: some field populations of a major cotton pest, Helicoverpa zea, evolved resistance to Cry1Ac, the Bacillus thuringiensis toxin (Bt) in first-generation transgenic Bt cotton (also called Bollgard cotton). This conclusion is based on extensive resistance monitoring data for 1992 to 2006 from five papers by Randall Luttrell and his collaborators2, 3, 4, 5, 6, including crucial information about field efficacy and larval survival on Bt cotton plants from three papers not cited by William Moar et al. above. These data show that the field-evolved resistance documented with laboratory diet bioassays (see Table 1 below) is associated with increased survival on Bt cotton leaves (Fig. 1) and control problems in the field2...(Continues in Journal )