Contribution of Bacillus thuringiensis cotton cultivars and insecticides to a fall armyworm, Spodoptera frugiperda (J. E. Smith), (Lepidoptera : Noctuidae) management strategy

Abstract

Fall armyworm, Spodoptera frugiperda (J. E. Smith), response to cottons, Gossypium hirsutum L., expressing Bacillus thuringiensis (Bt) insecticidal proteins, Cry1Ac (Bollgard®), Cry1Ac + Cry2Ab (Bollgard II®), and Cry1Ac + Cry1F (WideStrike™), was evaluated in field and laboratory experiments. In field trials, larvae that were infested on selected fruiting forms (squares, white flowers, and bolls) of WideStrike™ plants had lower survivorship and caused less injury than larvae on non-Bt plants, regardless of fruiting structures. Bollgard® and Bollgard II® plants produced no consistent negative effects on fall armyworm survivorship and injury. In no-choice laboratory assays, Bollgard II® and WideStrike™ cotton tissue reduced fall armyworm larval development and survivorship compared to those larvae offered non-Bt tissue. Fall armyworm preference for oviposition sites on non-Bt and Bt-expressing cotton plants was evaluated by releasing adults into isolation cages containing plants of a single cotton line. The distribution of egg masses on non-Bt, Bollgard®, Bollgard II®, and WideStrike™ cotton plants was similar with the majority of egg clusters observed on the abaxial (underside) leaf surfaces. The field performance of selected novel and standard insecticides was evaluated against fall armyworm in conventional non-Bt cotton, sprayed with recommended (full) rates of products, and in Bollgard II® cotton, sprayed with reduced (one-half) rates of the same products. Insecticide-treated terminal leaves and bolls were removed from plants in a field environment, placed in plastic dishes and infested with a single third instar. Reducing insecticide rates on Bollgard II® cotton did not negatively affect efficacy of any insecticide compared to efficacy of full rates applied to conventional non-Bt cotton. These results show differences between the currently available Bt cotton technologies in their performance against fall armyworm larvae. This information should be used by the cotton industry in the selection of the most appropriate Bt traits if fall armyworm is considered a prevalent pest. Furthermore, opportunities to reduce insecticide rates without sacrificing satisfactory efficacy against fall armyworm on Bollgard II® plants could reduce chemical control costs. To better characterize fall armyworm identification and injury symptomology, descriptions and photographs were compiled in a manner that should be useful to cotton pest managers and producers.