Besides genetics and bioecological factors: Life-stage and insecticide persistence contribute to the Dalbulus maidis resistance to insecticides

The surge in Dalbulus maidis (Hemiptera: Cicadellidae) population in Brazil has prompted a rise in insecticide use. Resistance to bifenthrin (MoA 3A) and imidacloprid (MoA 4A) was already reported. The life-stage exposed to insecticides may influence D. maidis resistance evolution. Insecticide spray targeting life-stages presentig discrimination in susceptibility between susceptible and resistant phenotypes accelerates resistance evolution. Resistance leads to more frequent spraying and higher doses, increasing insecticide persistence. High doses may initially suppress resistant insects. As insecticide persistence decays, discrimination intensifies, eliminating susceptible insects. This study aimed to comprehend the resistance evolution of D. maidis to bifenthrin and imidacloprid. Leaf-dip bioassay were conducted to construct concentration-mortality curves using susceptible (Sus-Lab), bifenthrin-resistant (Bif-R), and imidacloprid-resistant (Imida-R) strains, across different life-stages. Mortality was assessed after 48h (bifenthrin) and 72h (imidacloprid). Corn plants were sprayed with bifenthrin and imidacloprid at field-recommended rates using a modified Potter Tower. Untreated control were sprayed with distilled water. Third-instar nymphs and adults of Sus-Lab, Bif-R, Imida-R, and heterozygous (Hz-Imida and Hz-Bif) strains were infested in plants at 0, 1, 3, and 7 days after spray (DAS). Mortality was assessed after 96h. Resistance ratios (RRs) for imidacloprid were lower in Imida-R nymphs than adults. For bifenthrin, RRs were low in the first- and third-instar nymphs of Bif-R. Bifenthrin starts discriminating Bif-R and Hz-Bif from Sus-Lab since 0 DAS. In contrast, imidacloprid starts discriminating Imida-R and Hz-Imida from Sus-Lab at 3 DAS. These findings demonstrate that life-stage and insecticide persistence influence D. maidis resistance evolution to pyrethroids and neonicotinoids.