Susceptibility of field populations of sugarcane borer from non‐Bt and Bt maize plants to five individual Cry toxins

Abstract Sugarcane borer, Diatraea saccharalis (F.), is a major target of transgenic maize expressing Bacillus thuringiensis (Bt) proteins in South America and the US mid‐south region. Resistance development in target pest populations is a major threat to the sustainable use of Bt crops. In our field trials in 2009, a significant number of live borers and plant injury from D. saccharalis were observed in an experimental SmartStax? maize line. The objective of this study was to assess the relative susceptibility of two field populations of D. saccharalis collected from non‐Bt and Bt maize plants containing SmartStax? traits to five individual Cry proteins. The five Bt proteins included two proteins (Cry1A.105 and Cry2Ab2) that were expressed in SmartStax? maize plants and three other common Bt proteins (Cry1Aa, Cry1Ab and Cry1Ac) that were not produced in SmartStax?. Larval mortality and growth inhibition on Bt diet of the fourth generation after field collections were evaluated 7 days after release of neonates on the diet surface. The laboratory bioassays showed that 50% lethal concentration (LC₅₀) values for Cry1A.105 and Cry2Ab2 for the population originated from Bt plants were 3.55‐ and 1.34‐fold greater, respectively, than those of the population collected from non‐Bt plants. In contrast, relative to the population from non‐Bt plants, the LC₅₀ of the population sampled from Bt plants were 3.85‐, 2.5‐ and 1.64‐fold more sensitive to Cry1Aa, Cry1Ab and Cry1Ac, respectively. The results did not provide clear evidence to conclude that the observed field survival of D. saccharalis on Bt plants was associated with increased levels of resistance.