Bt-toxin susceptibility and hormesis-like response in the invasive southern armyworm (Spodoptera eridania)

Abstract

Bacillus thuringiensis (Bt) varieties of soybean and cotton have been widely adopted in South America, but defoliation by the southern armyworm Spodoptera eridania (Lepidoptera: Noctuidae) has increased in these crops. This issue may be linked to the low inherent susceptibility of the larvae to some Bt toxins. However, this hypothesis was not yet tested, nor was the potential sublethal effects of the toxins on larval development of southern armyworm. Here we determined the response of southern armyworm larvae exposed to Bt toxins representative of those used in soybean and cotton cultivars. In concentration-response bioassays, whereas a low median lethal concentration (LC50) value was found for Cry2Aa (11 ng/cm2), higher LC50 values were found for Cry1F (>3000 ng/cm2) and Cry1Ac (>10000 ng/cm2). Larval growth of the southern armyworm showed a hormesis-like response when the larvae were reared on artificial diet containing increasing Cry1Ac concentrations. Such enhanced larval growth was also observed when fresh foliage of Cry1Ac soybean or its lyophilized tissue in an artificial diet were fed to the larvae. Although the foliage of the dual-toxin Bt cotton producing Cry1Ac + Cry2Ab killed all larvae, its lyophilized leaf tissue caused no significant larval mortality when diluted 25 times. These results indicate that southern armyworm larvae have low inherent susceptibility to Cry1Ac and Cry1F toxins, while the Cry1Ac Bt toxin can enhance its larval growth. In addition, Cry1Ac + Cry2Ab Bt cotton may not meet the high-dose condition for the southern armyworm. These findings have important implications for pest management of the southern armyworm, which is discussed.