Abstract
Evolution of Helicoverpa armigera resistance to Bacillus thuringiensis (Bt) cotton producing Cry1Ac is progressing in northern China, and replacement of Cry1Ac cotton by pyramided Bt cotton has been considered to counter such resistance. Here, we investigated four of the eight conditions underlying success of the refuge strategy for delaying resistance to Cry1Ac+Cry2Ab cotton, a pyramid that has been used extensively against H. armigera outside China. Laboratory bioassays of a Cry2Ab‐selected strain (An2Ab) and a related unselected strain (An) reveal that resistance to Cry2Ab (130‐fold) was nearly dominant, autosomally inherited, and controlled by more than one locus. Strong cross‐resistance occurred between Cry2Ab and Cry2Aa (81‐fold). Weaker cross‐resistance (18‐ to 22‐fold) between Cry2Ab and Cry1A toxins was also present and significantly increased survival of An2Ab relative to An on cotton cultivars producing the fusion protein Cry1Ac/Cry1Ab or Cry1Ac. Survival on Cry1Ac+Cry2Ab cotton was also significantly higher in An2Ab than in An, showing that redundant killing on this pyramid was incomplete. Survival on non‐Bt cotton did not differ significantly between An2Ab and An, indicating an absence of fitness costs affecting this trait. These results indicate that a switch to three‐toxin pyramided cotton could be valuable for increasing durability of Bt cotton in China.
Highlights
We used laboratory experiments to better understand the risk of H. armigera resistance to Cry1Ac+Cry2Ab cotton in China, by assessing several of the conditions underlying success of the refuge strategy for delaying resistance to this pyramided Bacillus thuringiensis (Bt) crops
We considered four of the eight conditions (Carrière et al, 2015, 2016) affecting evolution of H. armigera resistance to Cry1Ac+Cry2Ab cotton in China: dominance of resistance to Cry2Ab, cross-resistance between Cry1Ac and Cry2Ab, extent of redundant killing, and fitness costs associated with Cry2Ab resistance
We investigated these conditions by measuring responses of a strain selected for resistance to Cry2Ab in the laboratory (An2Ab), a related unselected strain (An), and relevant crosses between these strains in bioassays involving artificial diets treated with Cry1A and Cry2A toxins and Bt cotton plants producing Cry1Ac, a fusion protein Cry1Ac/Cry1Ab, and Cry1Ac+Cry2Ab
Summary
Bacillus thuringiensis (Bt) cotton producing the toxin Cry1Ac has provided significant environmental and economic benefits since its introduction in China in 1997 for control of major lepidopteran pests (Huang, Rozelle, Pray, & Wang, 2002; Lu, Wu, Jiang, Guo, & Desneux, 2012; Wu & Guo, 2005; Wu, Lu, Feng, Jiang, & Zhao, 2008). We considered four of the eight conditions (Carrière et al, 2015, 2016) affecting evolution of H. armigera resistance to Cry1Ac+Cry2Ab cotton in China: dominance of resistance to Cry2Ab, cross-resistance between Cry1Ac and Cry2Ab, extent of redundant killing, and fitness costs associated with Cry2Ab resistance. We investigated these conditions by measuring responses of a strain selected for resistance to Cry2Ab in the laboratory (An2Ab), a related unselected strain (An), and relevant crosses between these strains in bioassays involving artificial diets treated with Cry1A and Cry2A toxins and Bt cotton plants producing Cry1Ac, a fusion protein Cry1Ac/Cry1Ab, and Cry1Ac+Cry2Ab. We investigated the genetic basis of resistance to Cry2Ab (number of loci affecting resistance, maternal effects, and sex linkage) in the An2Ab strain
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