Early Warning of Cotton Bollworm Resistance Associated with Intensive Planting of Bt Cotton in China

Abstract

Transgenic crops producing Bacillus thuringiensis (Bt) toxins kill some key insect pests, but evolution of resistance by pests can reduce their efficacy. The predominant strategy for delaying pest resistance to Bt crops requires refuges of non-Bt host plants to promote survival of susceptible pests. To delay pest resistance to transgenic cotton producing Bt toxin Cry1Ac, farmers in the United States and Australia planted refuges of non-Bt cotton, while farmers in China have relied on “natural” refuges of non-Bt host plants other than cotton. Here we report data from a 2010 survey showing field-evolved resistance to Cry1Ac of the major target pest, cotton bollworm (Helicoverpa armigera), in northern China. Laboratory bioassay results show that susceptibility to Cry1Ac was significantly lower in 13 field populations from northern China, where Bt cotton has been planted intensively, than in two populations from sites in northwestern China where exposure to Bt cotton has been limited. Susceptibility to Bt toxin Cry2Ab did not differ between northern and northwestern China, demonstrating that resistance to Cry1Ac did not cause cross-resistance to Cry2Ab, and implying that resistance to Cry1Ac in northern China is a specific adaptation caused by exposure to this toxin in Bt cotton. Despite the resistance detected in laboratory bioassays, control failures of Bt cotton have not been reported in China. This early warning may spur proactive countermeasures, including a switch to transgenic cotton producing two or more toxins distinct from Cry1A toxins.

Potential shortfall of pyramided transgenic cotton for insect resistance management

“…Evolution of resistance by pests, however, is the most serious threat to the continued efficacy of Bt crops. Significant increases in the frequency of alleles conferring resistance to Bt toxins produced by transgenic crops have been reported in some populations of at least seven target species (4)(5)(6)(7)(8)(9)(10)(11)(12). Analyses of monitoring data and field experiments suggest that refuges of host plants that do not produce Bt toxins and grow near Bt crops can reduce the risk of resistance (4,5,13).…”

Efficacy of genetically modified Bt toxins against insects with different genetic mechanisms of resistance

“…[2][3][4][5][6] Many insects have been selected for resistance to Bt toxins in the laboratory, and some populations of at least eight crop pests have evolved resistance to Bt toxins outside of the laboratory, including two species resistant to Bt sprays and at least six species resistant to Bt crops. [2][3][4][5][6][9][10][11][12][13] The most widely used Bt toxins are crystalline proteins in the Cry1A family, particularly Cry1Ab in transgenic Bt corn and Cry1Ac in transgenic Bt cotton, which kill some lepidopteran larvae. 3 Cry1A toxins bind to the extracellular domains of cadherin, aminopeptidase, and alkaline phosphatase in larval midgut membranes.…”

Effects of Entomopathogenic Nematodes on Evolution of Pink Bollworm Resistance to <I>Bacillus thuringiensis</I> Toxin Cry1Ac

“…Field-evolved resistance to a toxin is deÞned as a genetically based decrease in susceptibility of a population to a toxin caused by exposure to that toxin in the Þeld ). Although Bt crops have remained effective for more than a decade against many pest populations, Þeld-evolved resistance to Bt crops has been reported in several cases (Van Rensburg 2007, Downes et al 2010, Storer et al 2010, Dhurua and Gujar 2011, Zhang et al 2011). …”