Abstract
Background
Spodoptera frugiperda
(J.E. Smith) is a highly polyphagous, migratory pest that causes considerable economic losses in maize worldwide. Transgenic Bt maize hybrids, particularly those pyramiding Cry and Vip3Aa20 proteins, are widely deployed against this pest, but prolonged field exposure can select for resistance. Resistance monitoring typically relies on neonate bioassays, yet field infestations comprise larvae of heterogeneous ages, and later instars may tolerate Bt tissue. We assessed the susceptibility of five geographically distinct
S. frugiperda
populations from the MATOPIBA region, each from areas under more than five years of continuous Bt maize cultivation across four larval instars.
Results
Time–mortality assays using F1 larvae of the first to fourth instars fed Bt or non‐Bt maize leaf tissue revealed a significant three‐way interaction among larval stage, population, and Bt hybrid. First‐ to third‐instar larvae were highly susceptible (near‐complete mortality within about five days) irrespective of population or hybrid, with a single marginal exception. In contrast, fourth‐instar larvae from the Bahia‐LE and Piauí‐Ur populations survived longer on some pyramided hybrids (up to 51.7% survival on DKB390PRO4). No consistent decline in susceptibility with larval age was observed across all locations, indicating a stage‐dependent effect expressed only in specific population–hybrid combinations.
Conclusion
Susceptibility of
S. frugiperda
to Cry/Vip3Aa20 Bt maize is jointly governed by larval stage, population, and hybrid. Because neonate‐only assays would have missed the late‐instar survival detected here, resistance‐monitoring protocols should incorporate multistage bioassays that include third‐ and fourth‐instar larvae. The localized late‐instar tolerance observed in high‐exposure populations does not by itself demonstrate field‐evolved resistance, but it constitutes an early‐warning signal that should trigger reinforced integrated resistance management refuge compliance, rotation of Bt modes of action, and regionally coordinated surveillance to preserve the durability of pyramided Bt maize.