Rachiplusia nu (Guenée) with field-evolved resistance to Bt soybean is susceptible to Bt-based biopesticides

Abstract Field-evolved resistance to the soybean event MON 87701 × MON 89788, which expresses the Cry1Ac toxin from Bacillus thuringiensis ( Bt ), was reported in Rachiplusia nu (Guenée) (Lepidoptera: Noctuidae) in Brazil and Argentina. Given this scenario, diet-overlay bioassays were conducted to assess the susceptibility of Brazilian populations of R. nu to the Bt -based insecticides DiPel WP and XenTari WG and to investigate potential cross-resistance with Cry1Ac using a laboratory-selected R. nu strain. Additionally, leaf bioassays were used to test the efficacy of field doses of these biopesticides against field-collected populations and laboratory-selected strains of R. nu. The LC 50 values of DiPel WP and XenTari WG for field populations of R. nu ranged from 0.03 to 0.09 and 0.46–0.60 μg active ingredient cm –2 of diet, respectively, representing resistance ratios ≤ 4.0-fold relative to a susceptible population. The Cry1Ac-resistant, Cry1Ac-susceptible and F 1 progeny of reciprocal crosses (heterozygotes) showed relatively similar susceptibility to DiPel WP and XenTari WG (resistance ratios ≤ 5.3-fold), indicating low but detectable cross resistance . Leaf bioassays demonstrated that field doses of DiPel WP and XenTari WG caused > 95% mortality in all populations and strains. These results indicate that Brazilian populations of R. nu remain susceptible to DiPel WP and XenTari WG, even after the evolution of resistance to Cry1Ac soybean and highlight these products as effective tools for controlling this polyphagous pest.