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 LC₅₀ of AcMNPV ranged from 1.9 × 10⁷ to 7.9 × 10⁷ occlusion bodies (OBs)/mL, indicating a low (< 4.2-fold) interpopulation variation in susceptibility. The diagnostic concentration of AcMNPV established, based on the calculated LC₉₉ (1.6 × 10⁹ 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.