Bt-induced hormesis in Bt-resistant insects: Theoretical possibility or factual concern?

The biphasic dose-response of a stressor where low amounts of a toxicant may stimulate some biological processes is a recent focus of attention in insecticide ecotoxicology. Nonetheless, the importance and management consequences of this phenomenon of pesticide-induced hormesis remain largely unrecognized. Curiously, the potential induction of hormesis by insecticidal proteins such as Bacillus thuringiensis toxins (i.e., Bt toxins), a major agriculture pest management tool of widespread use, has been wholly neglected. Thus, we aimed to circumvent this shortcoming while assessing the potential occurrence of hormesis induced by the Bt toxin Cry1Fa in its main target pest species - the fall armyworm Spodoptera frugiperda. Concentration-response bioassays were carried out in a Bt-susceptible and a Bt-resistant population providing the purified Cry1Fa toxin in artificial diet and recording the insect demographic parameters. As significant hormetic effect was detected in both populations with a significant increase in the net reproductive rate and the intrinsic rate of population growth, the potential occurrence of Bt-induced hormesis was subsequently tested providing the insects with leaves from transgenic Bt maize expressing the toxic protein. The performance of the Bt-resistant insects was not different in both maize genotypes, indicating that the leaf expression of the Bt protein did not promote hormesis in the resistant insects. Thus, despite the Bt-induced hormesis detected in the purified protein bioassays, the phenomenon was not detected with current levels of Bt expression in maize minimizing the risk of this additional efficacy constraint besides that of field occurrence of Bt resistance.

Fitness costs and stability of Cry1Fa resistance in Brazilian populations of Spodoptera frugiperda.

BACKGROUND: The presence of fitness costs of resistance to Bacillus thuringiensis (Bt) insecticidal proteins in insect populations may delay or even reverse the local selection of insect resistance to Bt transgenic crops, and deserves rigorous investigation. Here we assessed the fitness costs associated with Cry1Fa resistance in two strains of fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae), derived from field collections in different Brazilian regions and further selected in the laboratory for high levels of resistance to Cry1Fa using leaves of TC1507 corn. RESULTS: Fitness components were compared using paired resistant and susceptible strains with similar genetic backgrounds and F1 generations from reciprocal crosses, all of them reared on non-transgenic corn leaves. No apparent life history costs in the larval stage were observed in the Bt-resistant strains. Moreover, the resistance remained stable for seven generations in the absence of selection, with no decrease in the proportion of resistant individuals. Larval respiration rates were also similar between resistant and susceptible homozygotes, and heterozygotes displayed respiration rates and demographic performance equal or superior to those of susceptible homozygotes. CONCLUSION: In combination, these results indicate the lack of strong fitness costs associated with resistance to Cry1Fa in the fall armyworm strains studied. These findings suggest that Cry1Fa resistance in S. frugiperda populations is unlikely to be counterselected in Cry1Fa-free environments. © 2016 Society of Chemical Industry.

Resistance to dual-gene Bt maize in Spodoptera frugiperda: selection, inheritance, and cross-resistance to other transgenic events.

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.