Although we share the general concerns about pest resistance to transgenic crops discussed by F. Huang et al. [Reports, "Inheritance of resistance to Bacillus thuringiensis toxin (Dipel ES) in the European corn borer," 7 May 1999, p. 965], reconsideration of several aspects of their report is warranted. They examined resistance to Dipel ES, a commercial formulation of B. thuringiensis (Bt) toxin, in a laboratory-selected strain of the European corn borer, a major lepidopteran pest. Bt genes encoding insecticidal proteins have been introduced into the genome of maize to provide protection from larvae of the European corn borer. The transgenic hybrids are called Bt maize.

The results presented by Huang et al., however, are not directly relevant to potential resistance of the European corn borer to Bt maize because Dipel ES differs substantially from the toxins produced by Bt maize. Dipel ES contains Bt spores and at least three Bt toxins (Cry1Aa, Cry2A, and Cry2B) that are not present in Bt maize. Thus, the European corn borer strain studied by Huang et al. could be resistant to these components of Dipel ES, rather than to the toxins in Bt maize.

Huang et al. mention in note 7 that neonates from their European corn borer strain with 65-fold resistance to Dipel ES caused more damage than susceptible insects when placed on certain Bt maize hybrids. However, damage by neonates is not a reliable indicator of survival on transgenic plants. Results with the Colorado potato beetle show that neonates with greater than 400-fold resistance to Bt toxin Cry3A do not survive on Bt potato plants that produce Cry3A (1). Therefore, in some cases, pests may need extremely high levels of resistance to overcome the high concentrations of toxin in Bt plants.

The critical point about the inheritance of resistance and its implications for resistance management is whether heterozygotes die on transgenic plants. Huang et al. provide no evidence that either larvae from their Dipel ES-resistant strain or heterozygous larvae can survive to maturity on Bt maize, which means that no conclusions can be drawn about inheritance of resistance to Bt maize.

In contrast to survival of resistant diamondback moth on Bt broccoli and Bt canola (2) and resistant pink bollworm on Bt cotton (3), as far as we know, no one has reported results showing survival of European corn borer on Bt maize. The failure to find such resistance in European corn borer despite extensive efforts (4) bodes well for managing resistance of this pest to Bt maize. Several examples of nonrecessive inheritance of resistance to Bt toxins are known (5), but in the few cases of resistance to Bt plants analyzed so far, inheritance of resistance to the Bt plants is recessive (3, 6).

Bruce E. Tabashnik
Department of Entomology,
University of Arizona,
Tucson, AZ 85750, USA.

Richard T. Roush
Waite Institute,
University of Adelaide,
Glen Osmond 5064,
South Australia, Australia

Elizabeth D. Earle
Department of Plant Breeding,
Cornell University,
Ithaca, NY 14853, USA

Anthony M. Shelton
Department of Entomology,
Cornell University,
New York State Agricultural Experiment Station,
Geneva, NY 14456, USA

References

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