Abstract
Transgenic crop “pyramids” producing two or more Bacillus thuringiensis (Bt) toxins active against the same pest are used to delay evolution of resistance in insect pest populations. Laboratory and greenhouse experiments were performed with fall armyworm, Spodoptera frugiperda, to characterize resistance to Bt maize producing Cry1A.105 and Cry2Ab and test some assumptions of the “pyramid” resistance management strategy. Selection of a field-derived strain of S. frugiperda already resistant to Cry1F maize with Cry1A.105 + Cry2Ab maize for ten generations produced resistance that allowed the larvae to colonize and complete the life cycle on these Bt maize plants. Greenhouse experiments revealed that the resistance was completely recessive (Dx = 0), incomplete, autosomal, and without maternal effects or cross-resistance to the Vip3Aa20 toxin produced in other Bt maize events. This profile of resistance supports some of the assumptions of the pyramid strategy for resistance management. However, laboratory experiments with purified Bt toxin and plant leaf tissue showed that resistance to Cry1A.105 + Cry2Ab2 maize further increased resistance to Cry1Fa, which indicates that populations of fall armyworm have high potential for developing resistance to some currently available pyramided maize used against this pest, especially where resistance to Cry1Fa was reported in the field.
Highlights
Generation of seletion (c) developmental time from neonate to adult for the selected S. frugiperda strain reared on maize leaves of the event MON89034 (–● –) or its non-transgenic isoline (–○ –) during 10 generations of selection for resistance to event MON89034, which produces Cry1A.105 and Cry2Ab proteins from B. thuringiensis
Larvae under exposure to the pyramided Bacillus thuringiensis (Bt) maize responded to selection for resistance to Cry1A.105 and Cry2Ab proteins, as indicated by increased survival, pupal biomass and a decrease in development time from neonate to adult during the selection experiment (Fig. 1)
Survival rates for individuals reared on transgenic maize increased until the fourth generation of selection, and kept constant above 35%; in the later generations of selection, the selected strain was surviving at rates similar to those of control individuals reared in non-Bt maize (P > 0.05) (Fig. 1a)
Summary
Generation of seletion (c) developmental time from neonate to adult for the selected S. frugiperda strain reared on maize leaves of the event MON89034 (–● –) or its non-transgenic isoline (–○ –) during 10 generations of selection for resistance to event MON89034, which produces Cry1A.105 and Cry2Ab proteins from B. thuringiensis. We found that the resistance to pyramided maize was completely recessive (Dx = 0), incomplete, autosomal, and without maternal effects or cross-resistance to the Vip3Aa20 toxin produced in other Bt maize events This profile is favorable for resistance management, selection for resistance to Cry1A.105 + Cry2Ab reduced the susceptibility to Cry1Fa, indicating cross-resistance between these toxins. This finding raises concern regarding the sustainable use and efficacy of pyramided maize hybrids in countries such as United States and Brazil, where the frequency of Cry1Fa resistance alleles in S. frugiperda is high,[16,20,21] and many of the pyramided maize hybrids available produce Cry1A, Cry1F, and Cry2A9
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