Is insecticide resistance a factor contributing to the increasing problems with Dalbulus maidis (Hemiptera: Cicadellidae) in Brazil?

Abstract Background The corn leafhopper, Dalbulus maidis , is an important pest in Brazil. While chemical control has traditionally been the cornerstone for managing this pest, field control failures have been reported for some insecticides. To understand if these failures are due to resistance, we evaluated the susceptibility of 11 field‐collected populations of D. maidis to major insecticides during the 2021–2022 crop seasons in Brazil using concentration‐mortality bioassays. Additionally, we employed diagnostic concentration bioassays and foliar sprays at label‐recommended rates in 8–10 populations collected during the 2022–2023 crop seasons. Results High susceptibility to methomyl, carbosulfan and acephate was observed on concentration‐mortality bioassays across all populations tested with resistance ratio (RR) based on LC 50 <10‐fold, except for one population from Bahia State that exhibited reduced susceptibility to methomyl (RR = 17.5). On the other hand, all populations exhibited reduced susceptibility to bifenthrin, acetamiprid, and imidacloprid, with RR ranging from 90 to 2000‐fold. This reduced susceptibility to neonicotinoid and pyrethroid insecticides was further confirmed at diagnostic concentrations based on LC 99 of the susceptible strain, with survival rates >20% and in foliar sprays with mortality rates <80%. Most populations exposed to acephate and carbosulfan exhibited low survival rates at diagnostic concentrations (<5%) and high mortality rates in foliar sprays (>80%). Conclusions The reduced susceptibility to pyrethroid and neonicotinoid insecticides likely explain the field failures in controlling D. maidis populations in Brazil. This study represents the first large‐scale susceptibility monitoring of D. maidis to insecticides, and the results will contribute to decision‐making regarding the management of this pest. © 2024 Society of Chemical Industry.