Susceptibility of Cry1Ab-susceptible and -resistant sugarcane borer to transgenic corn plants containing single or pyramided Bacillus thuringiensis genes

Multiple independent trials were conducted to evaluate the performance of Cry1Ab-susceptible (Cry1Ab-SS), -heterozygous (Cry1Ab-RS), and -resistant (Cry1Ab-RR) genotypes of the sugarcane borer, Diatraea saccharalis (F.), on eight commercial hybrids and six experimental corn lines. The commercial varieties included two non-Bt and six Bt corn hybrids that expressed a single Bt protein (either Cry1Ab or Cry1F) targeting above-ground lepidopteran pests. The six experimental lines consisted of two non-Bt and four Bt corn lines, two expressing just the Cry1Ab protein and two containing the pyramided-genes Cry1A.105 and Cry2Ab2 (event MON 89034). Larval mortality on non-Bt corn leaf tissue ranged from 6 to 45% after 12 d across insect genotypes. The 12 d mortality of Cry1Ab-SS on leaf tissue of commercial Cry1Ab or Cry1F corn was 96–100%, whereas it was 80–96% for Cry1Ab-RS and 68–78% for Cry1Ab-RR. On intact plants, 39–64% of larvae survived on non-Bt corn plants after 21–25 d. Larval survivorship on intact plants of commercial Cry1Ab or Cry1F corn was 0–8.1% for Cry1Ab-SS, 1.3–34% for Cry1Ab-RS, and 19–51% for Cry1Ab-RR. Larvae of Cry1Ab-RR and -RS also caused significant plant injury to most of the commercial Bt corn hybrids, especially to the Cry1Ab corn. Cry1Ab resistance in D. saccharalis was incompletely dominant on commercial Bt corn hybrids. However, both experimental lines with pyramided genes of Cry1A.105 and Cry2Ab2 provided complete control of all three insect genotypes in both leaf tissue and intact plant tests. Results of this study suggest that MON 89034 should offer a means for Bt resistance management in D. saccharalis.

Research highlights
► Pyramiding two or more dissimilar Bt genes into one plant has been identified as a potential IRM strategy for Bt crops. However, experimental data that can support the gene-pyramiding strategy have not been available for any insect pests targeted by Bt corn.
► Here we demonstrate that a Cry1Ab-resistant strain of sugarcane borer that can survive on commercial Cry1Ab corn hybrids also displays evident cross-resistance to Cry1F maize plants. However it cannot survive on a novel pyramided Bt maize event (MON 89034).
► Sugarcane borer is a major corn stalk-boring pest and a primary target of Bt maize in the U.S. mid-south region and South America.
► This paper presents the first demonstration of cross-resistance within commercial Bt corn events and provides the first showcase for how a significant resistance to Cry1Ab corn can be completely overcome by pyramided Bt corn.