Baseline Susceptibility and Diagnostic Concentration of AcMNPV in Rachiplusia nu (Lepidoptera: Noctuidae) in Brazil and Cross-resistance to Cry1Ac

The sunflower looper, Rachiplusia nu (Guenée, 1852) (Lepidoptera: Noctuidae), has evolved practical resistance to the Cry1Ac toxin from Bacillus thuringiensis Berliner ( Bt ) expressed in soybean in Brazil and neighboring countries. In view of this, the use of chemical or biological insecticides is required for their control. In 2023, a bioinsecticide based on Autographa californica multiple nucleopolyhedrovirus (AcMNPV: Baculoviridae: Alphabaculovirus) was registered for the control of R. nu in Brazil. To support the use of this new insecticide, we conducted diet-overlay bioassays to characterize the baseline susceptibility of Brazilian populations of R. nu to AcMNPV and establish a diagnostic concentration for resistance monitoring. Additionally, we evaluated cross-resistance between AcMNPV and Cry1Ac by testing AcMNPV in both Cry1Ac-susceptible and -resistant populations of R. nu . The tested field populations of R. nu were susceptible to AcMNPV. The LC 50 of AcMNPV ranged from 1.9 × 10 7 to 7.9 × 10 7 occlusion bodies (OBs)/mL, indicating a low (< 4.2-fold) interpopulation variation in susceptibility. The diagnostic concentration of AcMNPV established, based on the calculated LC 99 (1.6 × 10 9 OBs/mL), caused > 97% mortality in field populations of R. nu . No cross-resistance was detected between AcMNPV and Cry1Ac Bt toxin in this species. These findings indicate that the AcMNPV-based bioinsecticide may be a valuable tool in integrated management programs to control R. nu and that the diagnostic concentration determined here is suitable for resistance monitoring.