Development of resistance by fall armyworm (Spodoptera frugiperda) to Cry proteins in Bt maize

Abstract

The fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) has become one of the most damaging agricultural pests since it was first recorded outside of its native area of the Americas in 2016. In the Americas, genetically modified Bt maize is used as a control strategy for control of this pest. Although FAW is not one of the target pests of Bt maize in South Africa, it has suppressive effects on pest populations in areas where it occurs. Large scale cultivation of Bt maize exerts strong selection pressure on FAW populations, which subsequently evolve resistance to Bt toxins. The development of resistance to Bt proteins by FAW populations could lead to the unsuccessful implementation of Bt technologies, yielding them unreliant and unsustainable. In an attempt to mitigate this threat, single-toxin Bt cultivars were replaced by multi-toxin (pyramid) maize cultivars, and the deployment of specific insect resistance management (IRM) strategies. The most commonly used IRM strategy is the high-dose and refuge (HDR) strategy which is based on certain assumptions regarding the biology and lifecycle of target pests. In order for this HDR strategy to be successful, assumptions regarding the biology of target pests as well as insect-host plant interactions must be met. These assumptions require transgenic crops to express high doses of Bt toxins, which will cause the death of all susceptible (SS) and heterozygous resistant (RS) larvae, permitting only homozygous resistant (RR) individuals to complete lifecycles on Bt maize. Furthermore, this IRM strategy also requires the planting of a designated refuge area consisting of non-Bt maize to provide areas where populations of susceptible pests will be able to survive and reproduce. It is then assumed that Bt-resistant moths emerging from Bt maize will have the ability to fly and mate with susceptible moths emerging from refuge areas, diluting the resistance alleles and producing heterozygous resistant offspring, which will once again succumb to the high-dose produced by Bt maize. The aim of this study was to determine if changes occur in the life history parameters of S. frugiperda after exposure to Bt maize as host plant over different generations. The specific objectives of this research were to rear a field-collected FAW population on non-Bt maize, as well as MON810 and MON89034 maize cultivars for nine generations, and to determine if changes in life history parameters, larval preference and moth flight ability occur with continuous exposure over generations. Field collected FAW larvae were reared on three maize varieties (feeding groups), for eight or nine generations. These varieties were: a single-gene Bt maize hybrid (MON810) expressing Cry1Ab protein, a pyramid-gene Bt maize hybrid (MON89034) expressing Cry1A.105 + Cry2Ab2 proteins, and a non-Bt iso-hybrid. Results showed notable changes in life history parameters of larvae, pupae and moths reared on Bt plant tissue. Larval survival on Bt maize was significantly lower than on non-Bt maize and the duration of development period of larvae was significantly extended in the Bt maize feeding groups. Pupal mass was lower and the duration of the pupal period was longer in groups that were exposed to Bt maize. These extended development periods will result in poor synchronization of moth emergence patterns between Bt fields and refuge plantings, which will contribute to increased selection for resistance. Moth flight ability was adversely affected in the feeding groups reared on Bt maize. Adverse effects on moth flight ability could imply that resistant moths have limited capacity to fly and may be more likely to mate with other resistant moths that emerge from the same Bt maize field, increasing the selection for resistance evolution.