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

## TL;DR

This study shows that a new virus-based insecticide is effective against a pest in Brazil and does not face resistance from prior toxin exposure.

## Contribution

Established baseline susceptibility and a diagnostic concentration for AcMNPV in Rachiplusia nu and confirmed no cross-resistance to Cry1Ac.

## Key findings

- Brazilian Rachiplusia nu populations are susceptible to AcMNPV with low interpopulation variation.
- A diagnostic concentration of AcMNPV (1.6 × 10⁹ OBs/mL) caused over 97% mortality in field populations.
- No cross-resistance was found between AcMNPV and Cry1Ac Bt toxin in Rachiplusia nu.

## Abstract

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.