Cross-resistance of Cry1Ab-selected Asian corn borer to other Cry toxins
The Asian corn borer, Ostrinia furnacalis (Guenée) (Lepidoptera: Crambidae), is the most important insect pest of maize in China. Despite the proven track record of biotech Bacillus thuringiensis (Bt) maize to provide an effective means to control lepidopteran pests such as the European corn borer Ostrinia nubilalis (Hübner), these crops have not been commercialized in China. However, Cry1Ab- and Cry1F-expressing maize derived from genetic transformation events MON810 (Monsanto), Bt11 (Syngenta) and TC1507 (Pioneer, DuPont) as well as Cry1Ah-, Cry1A- (a Cry1Ab/Cry1Ac hybrid toxin), and Cry1Ac-, Cry1Ie-expressing maize developed by the Chinese institutions have been shown to effectively suppress O. furnacalis in field trials. The development of resistance in target pests is a major threat to the use of Bt maize. A strain of O. furnacalis selected with Cry1Ab protein incorporated into artificial diet developed more than 100-fold resistance to Cry1Ab after 35 generations of selection, and readily consumed Cry1Ab-expressing maize silks. Although LC50s of the selected insect strain were not progressively increased, and even decreased in the following generations, these insects (ACB-AbR) could survive on Cry1Ab-expressing maize tissue after 51 generations of selection. In addition, susceptibility of ACB-AbR to a number of other Bt toxins to which the selected strains had not previously been exposed, was significantly decreased. The highest level of cross-resistance was observed with Cry1Ah (131-fold), followed by Cry1Ac (36-fold). A low level of cross-resistance (6-fold) to Cry1F was also detected. In contrast, ACB-AbR was equally susceptible to Cry1Ie as the unselected control strain. These results indicate that the availability of multiple toxins could improve resistance management strategies, provided that the potential for cross-resistance among toxins is not an issue.