Abstract
Evolution of resistance by insect pests threatens the long-term benefits of transgenic crops that produce insecticidal proteins from Bacillus thuringiensis (Bt). Previous work has detected increases in the frequency of resistance to Bt toxin Cry1Ac in populations of cotton bollworm, Helicoverpa armigera, from northern China where Bt cotton producing Cry1Ac has been grown extensively for more than a decade. Confirming that trend, we report evidence from 2011 showing that the percentage of individuals resistant to a diagnostic concentration of Cry1Ac was significantly higher in two populations from different provinces of northern China (1.4% and 2.3%) compared with previously tested susceptible field populations (0%). We isolated two resistant strains: one from each of the two field-selected populations. Relative to a susceptible strain, the two strains had 460- and 1200-fold resistance to Cry1Ac, respectively. Both strains had dominant resistance to a diagnostic concentration of Cry1Ac in diet and to Bt cotton leaves containing Cry1Ac. Both strains had low, but significant cross-resistance to Cry2Ab (4.2- and 5.9-fold), which is used widely as the second toxin in two-toxin Bt cotton. Compared with resistance in other strains of H. armigera, the resistance in the two strains characterized here may be especially difficult to suppress.
Highlights
The insecticidal proteins of Bacillus thuringiensis (Bt) kill some major insect pests, but are harmless to vertebrates and most other organisms (Mendelsohn et al 2003; Sanahuja et al 2011; Pardo-Lopez et al 2013)
Because the refuge and pyramid strategies for delaying resistance require understanding of the dominance of resistance and cross-resistance, we evaluated these traits in two resistant strains of H. armigera isolated from populations in two provinces of northern China where Bt cotton has been grown extensively for more than a decade
After the diet cooled and solidified, 100 lL of phosphate-buffered solution (PBS) containing the desired concentration of Bt toxin was applied evenly to the diet surface in each well and allowed to air dry, and a single larva was placed in each well
Summary
The insecticidal proteins of Bacillus thuringiensis (Bt) kill some major insect pests, but are harmless to vertebrates and most other organisms (Mendelsohn et al 2003; Sanahuja et al 2011; Pardo-Lopez et al 2013). Corn and cotton plants genetically engineered to produce Bt toxins have provided many benefits including pest suppression, reduced use of insecticide sprays, conservation of natural enemies, increased yield, and higher farmer profits (Wu et al 2008; Carpenter 2010; Hutchison et al 2010; National Research Council 2010; Tabashnik et al 2010; Edgerton 2012; Kathage and Qaim 2012; Lu et al 2012). Field-evolved resistance associated with reduced efficacy of Bt toxins has been reported in some populations of seven pest species: two targeted by Bt sprays (Tabashnik et al 1990; Janmaat and Myers 2003) and five targeted by Bt crops (Luttrell et al 2004; Van Rensburg 2007; Tabashnik et al 2008, 2013; Storer et al 2010; Dhurua and Gujar 2011; Gassmann et al 2011). Other cases of significant decreases in susceptibility to the Bt toxins in transgenic crops including ‘incipient resistance’ and ‘early warning’ of resistance have been detected in at least four additional pest species
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