Cross-resistance to toxins used in pyramided Bt crops and resistance to Bt sprays in Helicoverpa zea

• Published in: Journal of invertebrate pathology Vol. 132; pp. 149 - 156
• Main Authors: Welch, Kara L., Unnithan, Gopalan C., Degain, Ben A., Wei, Jizhen, Zhang, Jie, Li, Xianchun, Tabashnik, Bruce E., Carrière, Yves
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01-11-2015
• Subjects: Animals; Bacillus thuringiensis; Bacterial Proteins - chemistry; Bt crops; Bt sprays; Bt toxin structure; Crops, Agricultural - genetics; Cross-resistance; Endotoxins - chemistry; Helicoverpa zea; Hemolysin Proteins - chemistry; Insecticide Resistance; Larva - drug effects; Lepidoptera; Moths - drug effects; Plants, Genetically Modified; Resistance management; Sequence Analysis, Protein; Bt toxin structure; Bt crops; Bt sprays; Cross-resistance; Resistance management
• ISSN: 0022-2011; 1096-0805
• DOI: 10.1016/j.jip.2015.10.003

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•We evaluated cross-resistance between Cry1Ac and Bt toxins and sprays in Helicoverpa zea.•Cross-resistance occurred between Cry1Ac and five Bt toxins used in pyramided Bt crops.•Cross-resistance between toxins and Cry1Ac was associated with amino acid sequence similarity of domain II.•Selection with Cry1Ac increased resistance to Bt sprays.•Cross-resistance may affect the evolution of resistance to Bt crops and sprays in H. zea.
To delay evolution of resistance by insect pests, farmers are rapidly increasing their use of transgenic crops producing two or more Bacillus thuringiensis (Bt) toxins that kill the same pest. A key condition favoring durability of these “pyramided” crops is the absence of cross-resistance between toxins. Here we evaluated cross-resistance in the major lepidopteran pest Helicoverpa zea (Boddie) to Bt toxins used in pyramids. In the laboratory, we selected a strain of this pest with Bt toxin Cry1Ac followed by selection with MVP II, a formulation containing a hybrid protoxin that is identical to Cry1Ac in the active portion of the toxin and 98.5% identical overall. We calculated the resistance ratio as the EC50 (concentration causing mortality or failure to develop beyond the first instar of 50% of larvae) for the laboratory-selected strain divided by the EC50 for its field-derived parent strain that was not selected in the laboratory. The resistance ratio was 20.0–33.9 (mean=27.0) for MVP II, 57.0 for Cry1Ac, 51.3 for Cry1A.105, 22.4 for Cry1Ab, 3.3 for Cry2Ab, 1.8 for Cry1Fa, and 1.6 for Vip3Aa. Resistance ratios were 2.9 for DiPel ES and 2.0 for Agree VG, which are commercial Bt spray formulations containing Cry1Ac, other Bt toxins, and Bt spores. By the conservative criterion of non-overlap of 95% fiducial limits, the EC50 was significantly higher for the selected strain than its parent strain for MVP II, Cry1Ac, Cry1A.105, Cry1Ab, Cry2Ab and DiPel ES. For Cry1Fa, Vip3Aa, and Agree VG, significantly lower susceptibility to a high concentration indicated low cross-resistance. The resistance ratio for toxins other than Cry1Ac was associated with their amino acid sequence similarity to Cry1Ac in domain II. Resistance to Cry1Ac and the observed cross-resistance to other Bt toxins could accelerate evolution of H. zea resistance to currently registered Bt sprays and pyramided Bt crops.