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.
Highlights
The toxins produced by Bacillus thuringiensis (Bt) kill some major insect pests, but cause little or no harm to vertebrates and most other organisms [1]
The Shawan population from northwestern China, which had no exposure to Bt cotton, had the lowest LC50 (13) (Fig. 2, Table S1)
The results reported here from three different sets of bioassay parameters show that susceptibility to Bt toxin Cry1Ac was significantly lower in 13 populations from northern China than in two populations from northwestern China
Summary
The toxins produced by Bacillus thuringiensis (Bt) kill some major insect pests, but cause little or no harm to vertebrates and most other organisms [1]. Bt toxins have been used in sprays for decades and in transgenic plants since 1996 [2]. Transgenic corn and cotton producing Bt toxins grew on more than 50 million hectares worldwide in 2009 [3]. Many insects have been selected for resistance to Bt toxins in the laboratory, and some populations of at least six crop pests have evolved resistance to Bt toxins outside of the laboratory, including two species with resistance to Bt sprays and four species with resistance to Bt crops [12,13,14]
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