Landscape configurations of refuge areas that delay the evolution of resistance to Bt sugarcane: an agent based modeling approach

Although transgenic crops expressing genes from the bacterium Bacillus thuringiensis (Bt) are considered to be an effective pest control method, reckless usage adds environmental pressure on a pest population to develop resistance to the protein over time. The use of small portions of non-Bt crop (refuge areas) limits the rate of resistance development. Strains of Bt sugarcane for the South African market are being developed, and a prerequisite to releasing such a product on the market is a recommendation on the size and layout of the refuge areas. In this article, an agent-based simulation model is used to test the effectiveness of different landscape configurations of refuge areas in Bt sugarcane against resistance development occurring in an associated lepidopteran pest population. Individual insects are modeled as agents on an underlying sugarcane field that can either be Bt or refugium. The model is applied to 2 hypothetical case studies, each focusing on a specific aspect of refugia planning. The first focuses on the size and distribution of refuge, and the second on the shape of the refuge. A conservative general recommendation of 30% per farm, planted in large blocks on farms, is made based on simulation results and what is currently known about the target pest species, to provide regulatory bodies as well as growers with a starting point on how to regulate and plan refuge areas in South African Bt sugarcane.

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 F₂ isoline families of D. saccharalis collected from three major sugarcane planting states (Goiás, Minas Gerais and São 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 F₂ 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 Goiás and Minas Gerais were relatively low at 0.0034 for Cry1Ab and 0.0045 to Cry1Ac. By contrast, RAFs for São 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 São 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 F₂ isoline families of D. saccharalis collected from three major sugarcane planting states (Goiás, Minas Gerais and São Paulo) in Brazil during 2019-2020 were screened for resistance to Cry1Ab and Cry1Ac sugarcane plants Resistance allele frequencies (RAFs) for the populations from Goiás and Minas Gerais were relatively low at 0.0034 for Cry1Ab and 0.0045 to Cry1Ac. By contrast, RAFs for the São 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 São 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. © 2022 Society of Chemical Industry.