Assessing the potential for insecticide cross‐resistance to a novel insecticide in the Colorado potato beetle ( <scp> <i>Leptinotarsa decemlineata</i> </scp> ) using phenotypic and transcript expression analysis

Abstract BACKGROUND Cross‐resistance in the Colorado potato beetle ( Leptinotarsa decemlineata ) threatens insecticide efficacy and sustainable pest management. While metabolic detoxification is often implicated in resistance, its role in driving cross‐resistance evolution to novel insecticides remains unclear. Here, we evaluated phenotypic and transcriptional responses to both the widely used neonicotinoid imidacloprid and the novel and recently registered insecticide isocycloseram across multiple L. decemlineata populations within the United States. We tested for pre‐existing genetic variation and expression patterns linked to detoxification pathways. RESULTS Several populations showed significant phenotypic resistance to imidacloprid, while all populations remained more susceptible to isocycloseram. However, positive correlations between the responses to imidacloprid and isocycloseram provide evidence for emerging phenotypic cross‐resistance. Transcript expression analysis revealed significant, but inconsistently patterned treatment group‐, population‐, and dose‐based responses. Inducible up‐regulation of transcript expression was observed only in the imidacloprid treated samples. CONCLUSION These findings support the presence of phenotypic cross‐resistance in L. decemlineata , however, transcriptomic analyses of a subset of metabolic detoxification genes did not reveal consistent expression patterns across populations, treatments, or doses, limiting definitive inference about the contribution of detoxification pathways to this response. Variation seen here in gene expression across populations underscores the complexity of resistance evolution and the need for further investigation into the mechanisms underlying cross‐resistance. © 2026 The Author(s). Pest Management Science published by John Wiley &amp; Sons Ltd on behalf of Society of Chemical Industry.