Resistance of Dalbulus maidis (Hemiptera: Cicadellidae) to methomyl: Inheritance, cross-resistance and stability

The corn leafhopper Dalbulus maidis (Hemiptera: Cicadellidae) is a major corn pest in Brazil. Field-evolved resistance of D. maidis to pyrethroid and neonicotinoid insecticides has been reported, and reduced susceptibility to methomyl, a carbamate, has been observed in some regions. To implement resistance management strategies to delay the evolution of methomyl resistance in D. maidis in Brazil, this study aimed to select a methomyl-resistant strain (Met-R) of D. maidis, characterize the inheritance of resistance, evaluate cross-resistance to other insecticides and investigate resistance stability under laboratory conditions. The susceptible reference strain (SUS) has been maintained without selection pressure for more than 10 years. A field-collected population of D. maidis was mass selected with methomyl to obtain the Met-R strain. Leaf-dip bioassays were conducted to characterize concentration–response curves. Reciprocal crosses between Met-R and SUS were performed to produce heterozygotes, followed by backcrosses with SUS. Resistance ratios to various insecticides were estimated using SUS as a reference. Resistance stability was evaluated throughout eight generations without selection, using cages with different proportions of Met-R and SUS strains. After 12 generations of selection, the resistance ratio to methomyl increased from 10- to 76-fold in Met-R. Inheritance was autosomal, incompletely dominant and polygenic. Cross-resistance was detected to acetamiprid, imidacloprid, bifenthrin, and lambda-cyhalothrin, but not to acephate, carbosulfan, dinotefuran, chlorfenapyr, isocycloseram, or buprofezin. Resistance to methomyl was unstable in the absence of selection. Our results provide useful insights for implementing resistance management strategies of D. maidis to methomyl.