Temporal Changes in Insecticide Susceptibility of Field-Collected Pollen Beetle (Brassicogethes aeneus) Populations in the Czech Republic

The pollen beetle, Brassicogethes aeneus, is a major pest of winter oilseed rape in Europe, and the development of insecticide resistance remains a key challenge for its management. This study evaluated recent changes in susceptibility of B. aeneus populations to lambda-cyhalothrin, etofenprox, acetamiprid and thiacloprid in the Czech Republic during 2020–2025. Adult beetles were collected annually from 14 agricultural localities and tested using standardized IRAC vial bioassays. Dose–response parameters (LC50 and LC90), relative susceptibility ratios, and mortality at recommended field doses were analyzed using weighted linear mixed-effects models. Susceptibility-related responses differed markedly among insecticides and years. Absolute LC values occupied different numerical ranges among compounds, while mortality at the recommended field dose indicated the highest efficacy for acetamiprid and thiacloprid and the lowest for lambda-cyhalothrin. Several LC- and RSR-based endpoints showed elevated values around 2021–2022, followed by lower values in later years, although the magnitude and direction of temporal changes differed among insecticides. Significant insecticide × year interactions further demonstrated compound-specific temporal trajectories. Locality contributed to variability in LC-based endpoints but explained only a limited proportion of total variance. Long-term monitoring provides valuable information for resistance management and supports the need for diversified insecticide use in oilseed rape pest control.