BACKGROUND: The highly polyphagous and invasive fall armyworm (FAW, Spodoptera frugiperda) can feed on different plant parts of host crops, damaging whorls and stalks in early maize growth stages. Systemic insecticide seed treatment (IST) could minimize this damage, although the residual efficacy may vary with the plant tissue damaged. Using damage rating scales and artificial infestation in controlled conditions, we determined the potential of IST against FAW attacking maize whorl leaves or the stalk base.RESULTS: Chlorantraniliprole, cyantraniliprole, or thiodicarb + imidacloprid IST similarly killed > 80% FAWs for 1 or 2 weeks after plant emergence depending on the plant tissue attacked. The residual efficacy (i.e. time after plant emergence sustaining > 80% larval mortality) lasted from the first to the eleventh day (VE-V3 maize growth stages), while for cutworm on the maize stalk base, it lasted 3-7 days after plant emergence (V1-V2 stages). In terms of damage, the ISTs lasted 15 days after emergence (V4 stage) for FAW on whorl leaves and 10 days (V3 stage) for FAW feeding on the stalk base. The larvae surviving on the seedtreated plants underwent sublethal effects in growth and development, reducing insect fitness.CONCLUSION: Diamide or carbamate + neonicotinoid seed treatments kill FAW larvae on maize whorls or stalks in favorable edaphoclimatic and insecticide-susceptibility conditions. The cumulative impacts of systemic IST on aboveground insect pests go beyond mortality. The ISTs studied can be valuable against FAW in maize, for instance, to help protect varieties that may not express sufficient insect resistance in maize early growth stages.
An important step to devise appropriate pest management strategies for armyworms (Lepidoptera: Noctuidae) in Bacillus thuringiensis Berliner (Bt) crops is to determine the lethal, sublethal, and parental effects of Bt toxins on target and nontarget pest species. Here we documented the susceptibility of black armyworm, Spodoptera cosmioides (Walker), to three Cry toxins and its life-history traits feeding on dual-toxin Bt cotton and an artificial diet containing sublethal concentrations of Cry1Ac. In concentration–response bioassays, black armyworm larvae showed low susceptibility to Cry toxins, with 853 ng/cm2 as the lowest value estimated for the median lethal concentration (LC50). The decreasing rank of toxicity was Cry1F, Cry2Aa, and Cry1Ac. Foliage of dual-toxin Bt cotton varieties (Cry1Ac + Cry1F and Cry1Ab + Cry2Ae) caused higher larval mortality than Cry1Ac-expressing cotton. Black armyworms showed reduced larval weight when growing on the Cry1Ac-treated diet, yet they reached adulthood and produced offspring. Interestingly, these larvae were grown on the control diet and showed reduced weight gain associated with the toxin exposure of the previous generation, indicating a parental effect of the exposure to Cy1Ac. The reduced larval weight was recovered in later instars, and there was no significant change in the population fitness of the parental armyworms or their offspring. To our knowledge, this is the first study documenting the parental effects of Bt toxins in insects. These results advance our understanding of potential responses of nontarget species when exposed to Bt toxins and contribute to design pest management programs for armyworms and other nontarget lepidopteran species exposed to Bt crops.
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