Abstract
Evolution of resistance in pests threatens the long-term efficacy of insecticidal proteins from Bacillus thuringiensis (Bt) used in sprays and transgenic crops. Previous work showed that genetically modified Bt toxins Cry1AbMod and Cry1AcMod effectively countered resistance to native Bt toxins Cry1Ab and Cry1Ac in some pests, including pink bollworm (Pectinophora gossypiella). Here we report that Cry1AbMod and Cry1AcMod were also effective against a laboratory-selected strain of pink bollworm resistant to Cry2Ab as well as to Cry1Ab and Cry1Ac. Resistance ratios based on the concentration of toxin killing 50% of larvae for the resistant strain relative to a susceptible strain were 210 for Cry2Ab, 270 for Cry1Ab, and 310 for Cry1Ac, but only 1.6 for Cry1AbMod and 2.1 for Cry1AcMod. To evaluate the interactions among toxins, we tested combinations of Cry1AbMod, Cry1Ac, and Cry2Ab. For both the resistant and susceptible strains, the net results across all concentrations tested showed slight but significant synergism between Cry1AbMod and Cry2Ab, whereas the other combinations of toxins did not show consistent synergism or antagonism. The results suggest that the modified toxins might be useful for controlling populations of pink bollworm resistant to Cry1Ac, Cry2Ab, or both.
Highlights
The insecticidal proteins of Bacillus thuringiensis (Bt) kill some major insect pests, but are harmless to vertebrates and most other organisms [1,2,3]
The BX-R strain of pink bollworm, which we selected in the laboratory with Cry1Ac and Cry2Ab, was highly resistant to native Bt toxins, but not to Cry1AbMod and Cry1AcMod (Table 1)
For the native toxins Cry1Ab, Cry1Ac and Cry2Ab tested at 10 μg toxin per ml diet, survival was 81 to 100% for the resistant strain compared with 0% for the susceptible strain (Table 1)
Summary
The insecticidal proteins of Bacillus thuringiensis (Bt) kill some major insect pests, but are harmless to vertebrates and most other organisms [1,2,3]. Extensive use of Bt toxins in sprays and transgenic crops has caused field-evolved resistance in some pests, which entails a genetically based decrease in susceptibility [5,6,7,8]. Field-evolved resistance associated with reduced efficacy of Bt toxins has been reported in seven pest species, two targeted by Bt sprays [9,10] and five targeted by Bt crops [11,12,13,14,15,16]. 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 [8,17,18,19,20,21]
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