Varied frequencies of resistance alleles to Cry1Ab and Cry1Ac among Brazilian populations of the sugarcane borer, Diatraea saccharalis (F.)

BACKGROUND Brazil is the largest grower of the world's 26 million ha of sugarcane, Saccharum officinarum. Pest damage mainly by the sugarcane borer, Diatraea saccharalis (F.), is a great challenge to the sugarcane industry. To control D. saccharalis, Brazil launched the world's first commercial use of Bt sugarcane in 2017. As part of the resistance management programs for Bt sugarcane planting, 535 F-2 isoline families of D. saccharalis collected from three major sugarcane planting states (Goias, Minas Gerais and Sao Paulo) in Brazil during 2019-2020 were screened for resistance to two Bt sugarcane varieties: CTC20BT expressing Cry1Ab and CTC9001BT expressing Cry1Ac. Here we report the results of the first study related to Bt resistance in a sugarcane cropping system. RESULTS Larval survivorships of these families in an F-2 screen on CTC20BT were highly correlated with their survival on CTC9001BT, whereas the Cry1Ac tissues exhibited greater insecticidal activities than Cry1Ab. Resistance allele frequencies (RAFs) for populations from Goias and Minas Gerais were relatively low at 0.0034 for Cry1Ab and 0.0045 to Cry1Ac. By contrast, RAFs for Sao Paulo populations were considerably greater (0.0393 to Cry1Ab, 0.0245 to Cry1Ac). CONCLUSIONS RAFs to Cry1Ab and Cry1Ac varied among Brazilian D. saccharalis populations. Prior selection resulting from an intensive use of single-gene Bt maize under low compliance of refuge planting could be a main factor contributing to the high RAF in Sao Paulo. The results suggest that mitigation measures including sufficient non-Bt maize refuge planting, effective resistance monitoring, and use of pyramided Bt sugarcane traits should be implemented promptly to prevent further increase in the RAF to ensure the sustainable use of Bt sugarcane in Brazil. Mini abstract To control Diatraea saccharalis, Brazil launched the world's first commercial use of Bt sugarcane in 2017. As part of the resistance management programs for Bt sugarcane planting in Brazil, 535 F-2 isoline families of D. saccharalis collected from three major sugarcane planting states (Goias, Minas Gerais and Sao Paulo) in Brazil during 2019-2020 were screened for resistance to Cry1Ab and Cry1Ac sugarcane plants Resistance allele frequencies (RAFs) for the populations from Goias and Minas Gerais were relatively low at 0.0034 for Cry1Ab and 0.0045 to Cry1Ac. By contrast, RAFs for the Sao Paulo populations were considerably greater (0.0393 to Cry1Ab, 0.0245 to Cry1Ac). Prior selection resulting from an intensive use of single-gene Bt maize under low compliance of non-Bt maize refuge planting could be a main factor contributing to the high RAF in Sao Paulo. The results suggest that effective mitigation measures including sufficient non-Bt maize refuge planting, effective resistance monitoring and use of pyramided Bt sugarcane traits should be implemented promptly to prevent further increase in the RAF to ensure the sustainable use of Bt sugarcane in Brazil. (c) 2022 Society of Chemical Industry.

Automated summary

Brazil is the largest grower of sugarcane in the world and has faced significant challenges from the sugarcane borer. In 2017, Brazil introduced Bt sugarcane as a commercial solution to control the sugarcane borer. This study evaluated the resistance of different populations of the sugarcane borer in Brazil to Cry1Ab and Cry1Ac Bt sugarcane varieties, and found variations in resistance allele frequencies among different regions.