Susceptibility of Spodoptera frugiperda (Lepidoptera : Noctuidae) to Bt maize in South Africa

Abstract

Spodoptera frugiperda, the fall armyworm (FAW), invaded Africa during 2016 and is now considered the number one maize pest in Africa. The destructive feeding habits of FAW larvae threaten maize production in Africa. Bt maize is effective against African stem borer species and is expected to be approved for control of these pests in several African countries. Bt maize that express Cry proteins have been used effectively for control of the FAW in the United States, Canada, and several countries in South America. Although most Cry proteins provide effective control of the FAW, this pest evolved resistance to Cry1F Bt maize in Puerto Rico, Brazil and United States, and Cry1Ab Bt maize in Brazil. Proactive management of resistance evolution requires continued monitoring studies. The aim of this study was to provide baseline data on the control efficacy of Bt maize and the frequency of resistance alleles in field populations of S. frugiperda to single- and pyramid-gene Bt maize in South Africa. In order to determine the efficacy of Bt maize for the control of FAW a phenotypic screen was conducted and nine populations of S. frugiperda were evaluated, including a laboratory reared reference population. Larval feeding bioassays were conducted in which plant tissue of maize expressing Cry1Ab (single-toxin event) or Cry1A.105 + Cry2Ab2 (pyramid-toxin event), were fed to larvae. Results indicated moderate levels of survival (4-35%) on Cry1Ab maize, which supports field observations of commercial level control provided by this event. Considering Cry1A.105 + Cry2Ab2 maize, very high levels of mortality occurred with only one larvae being able to complete its life cycle. Although survival is low and effective control will definitely be achieved, resistance alleles seemed to be present and a genotypic evaluation was therefore done during 2019. During the second part of this study, a F2 screen was conducted to estimate frequency of resistant alleles and 117 families were established of two different field collected populations. Three of the 117 established families carried major resistance alleles against Cry1A.105 + Cry2Ab2 maize, with a low overall estimated frequency of 0.0084 (95% credibility interval of 0.0023 - 0.0181). The frequency of Cry1Ab resistance alleles was 0.0819 (95% credibility interval of 0.0617 - 0.1036). The high frequency of resistance alleles and moderate susceptibility of S. frugiperda to Cry1Ab could be ascribed to the latter being a low-dose event for this pest, as well as the fact that the individuals which initially arrived on the continent may have carried resistance alleles. This study provides base-line data regarding resistance of FAW in South Africa to single- and pyramid-gene Bt maize. Results include at what frequency resistance alleles occur naturally, how effective single- and pyramid-gene Bt maize are in controlling this pest and what effect Bt maize has on the life history parameters of the resistant individuals. These results predict that single-gene Bt maize will only provide short term control of this pest in Africa, and that pyramid-gene Bt maize will be more effective and sustainable within the parameters of IRM strategies to control this pest. We therefore advise that pyramid-gene Bt maize should be commercialized and that single-gene Bt maize should be retracted due to possible enhanced resistance development if these two events are cultivated simultaneously.