Relationships between Cry1Ac and Cry2Ab protein expression in field-grown Bollgard II (R) cotton and efficacy against Helicoverpa armi

摘要

Seven-day laboratory bioassays with first-instar Helicoverpa armigera and Helicoverpa punctigera larvae were conducted using leaves from field-grown Bollgard II (R) cotton during the 2009/10 season and larval mortality and development recorded. Leaves were from three farms in the St George region and two fields on one farm at Emerald in eastern Australia. The Cry1Ac and Cry2Ab content of leaves from the same samples was determined (see Knight et al., 2013) and the relationships between Cry protein content and larval mortality and development were examined. The Cry1Ac protein content was between 3.98 and 12.08 mu g/g during the growing season, while Cry2Ab content ranged between 300.6 and 9533 mu g/g. CrylAc and Cry2Ab content of leaves were highly correlated (r = 0.8276, P < 0.001). Seven-day mortality of H. punctigera larvae was close to 100% throughout the season. H. armigera mortality was close to 100% early in the season, but fell to similar to 65% by mid-to late February in the laboratory bioassays. Fitting three-dimensional non-linear models associating CrylAc and Cry2Ab content with H. armigera and H. punctigera mortality elucidated the relative importance of the two proteins in determining larval mortality; for this analysis, data were pooled with data from an isoline study to provide better sampling of the three-dimensional surface being modelled. For both Helicoverpa species, the fitted mortality response to the CrylAc protein was close to its maximum at protein concentrations above similar to 3 mu g/g Cry1Ac. For H. punctigera, response to the Cry2Ab protein was close to maximal once Cry2Ab was greater than similar to 200 mu g/g. In contrast, the fitted H. armigera mortality response to Cry2Ab increased steadily with concentration up to similar to 1200 mu g/g Cry2Ab. These responses led to markedly different response surfaces for the two species; H. punctigera mortality was close to 100% at most places on the response surface, while for H. arrnigera the response surface showed stronger increases in mortality with concentration for Cry2Ab than for Cry1Ac. These results can be interpreted as meaning that at the plant-expressed range of concentrations in Bollgard II cotton the two proteins are approximately equally important for H. punctigera but that changes in Cry2Ab content more strongly influences changes in larval mortality in H. armigera than does Cry1Ac, with CrylAc contributing a consistent 40-45% mortality for concentrations above 3 mu g/g. For H. armigera, there was no evidence of either synergism or antagonism between Cry1Ac and Cry2Ab proteins (P > 0.05) but this aspect was not testable for H. punctigera because mortality was mostly close to 100%. (C) 2015 Elsevier Ltd. All rights reserved.