Response of field populations and Cry-resistant strains of fall armyworm to Bt maize hybrids and Bt-based bioinsecticides

Abstract

Fall armyworm (FAW), Spodoptera frugiperda, is one of the most destructive pests of maize in warm regions of the Americas and a severe invasive species in Africa. Considering the armyworm management and its ability to develop resistance rapidly to synthetic insecticides and Bacillus thuringiensis (Bt) toxins, we assessed the response of field and laboratory populations of FAW to Bt maize hybrids and Bt-based bioinsecticides. In the laboratory, FAW neonates from two Bt-resistant colonies and three field-collected populations were fed foliage of Bt maize or Bt bioinsecticides incorporated into artificial diet, and larval survival and biomass were recorded after seven days. Larvae of the two laboratory colonies resistant to Bt maize producing Cry1 or Cry2 toxins had high larval survival rates on foliage of specific Bt maize hybrids (i.e., Cry1F or Cry1A.105 + Cry2Ab). On foliage of the Cry1A.105 + Cry2Ab Bt maize, larvae of one of the field-collected populations had survival rates similar to the Bt-resistant laboratory larvae but had strong growth inhibition. The field-collected larvae had less than 30% survival on foliage of Cry1F maize and larval weights equivalent to those of the Cry1F-resistant colony. In addition, while some larvae survived exposure to formulations of B. thuringiensis kurstaki (i.e., Dipel and Thuricide), the two formulations of B. thuringiensis aizawai (i.e., Agree and Xentari) caused 100% larval mortality in all FAW colonies and the field populations, including the ones showing reduced-susceptibility to Cry1F or Cry1A.105 + Cry2Ab Bt maize. This research shows that there are field populations of FAW with different levels of susceptibility to specific Bt toxins and that resistance to Cry1 and/or Cry2 may or may not reduce the efficacy of certain Bt bioinsecticides against the neonates.