Dose Effects of Flubendiamide and Thiodicarb against Spodoptera Species Developing on Bt and Non-Bt Soybean

SIMPLE SUMMARY: Previous studies have reported that infestations by species of the genus Spodoptera (Lepidoptera: Noctuidae) have increased in Cry1Ac Bt soybean fields in Brazil, indicating that chemical insecticides will be needed to reduce crop damage. For that reason, we evaluated the dose effects of the insecticides flubendiamide and thiodicarb against Spodoptera species surviving on soybean expressing Cry1A.105, Cry2Ab2 and Cry1Ac Bt proteins; soybean expressing just Cry1Ac; and non-Bt soybean. Our results indicated that L1 and L2 larvae of S. cosmioides, S. eridania and S. albula surviving on Cry1A.105/Cry2Ab2/Cry1Ac and Cry1Ac soybean leaves sprayed with 50% or 100% of the field label dose of flubendiamide (70 mL/ha) or thiodicarb (400 g/ha) presented >86% mortality. Among the Spodoptera larvae surviving insecticide treatment, only some of the S. frugiperda larvae surviving on Cry1Ac and non-Bt soybean sprayed with 50% of the field label dose of flubendiamide or thiodicarb produced fertile adults. However, the surviving insects had a longer larval stage duration and the females laid fewer eggs, indicating substantial sublethal effects on the biological traits of this species. ABSTRACT: An increase in Spodoptera species was reported in Bt soybean fields expressing Cry1Ac insecticidal proteins in Brazil, requiring additional management with chemical insecticides. Here, we evaluated the dose effects of flubendiamide and thiodicarb on Spodoptera cosmioides (Walker, 1858), Spodoptera eridania (Stoll, 1782), Spodoptera albula (Walker, 1857) and Spodoptera frugiperda (J. E. Smith, 1797) (Lepidoptera: Noctuidae) that survived on MON 87751 × MON 87708 × MON 87701 × MON 89788, expressing Cry1A.105, Cry2Ab2 and Cry1Ac; MON 87701 × MON 89788 soybean, expressing Cry1Ac; and non-Bt soybean. On unsprayed Cry1A.105/Cry2Ab2/Cry1Ac soybean, only S. frugiperda showed ~60% mortality after 10 d, whereas S. cosmioides, S. eridania and S. albula showed >81% mortality. The surviving larvae of all species on this Bt soybean showed >80% mortality when exposed to the field label dose of flubendiamide (70 mL/ha) or thiodicarb (400 g/ha) or at 50% of these doses. In contrast, all four species had <25% and <19% mortality on Cry1Ac and non-Bt soybean, respectively. The surviving S. cosmioides, S. eridania and S. albula on these soybean types presented >83% mortality after exposure to both dose levels of flubendiamide and thiodicarb. Some S. frugiperda larvae surviving on Cry1Ac and non-Bt soybean sprayed with a 50% dose of either insecticide developed into adults. However, the L1 larvae developing on Cry1Ac soybean leaves sprayed with flubendiamide and the L2 larvae on this soybean sprayed with thiodicarb had a prolonged immature stage, and the females displayed lower fecundity, which are likely to impact S. frugiperda population growth on soybean.