Sublethal insecticide exposure and inbreeding impose a developmental bottleneck but enhance crop-damaging traits in surviving Colorado potato beetles

Abstract Intensive agricultural insecticide use often fails to eradicate pest populations completely, leaving small remnant populations exposed to persistent sublethal residues and potentially elevated inbreeding. How such remnant populations persist remains poorly understood. Here, we examined how inbreeding and sublethal imidacloprid exposure shape survival and post-emergence performance in the Colorado potato beetle (Leptinotarsa decemlineata), a major agricultural pest. We compared four groups differing in breeding status and insecticide exposure history, and quantified emergence rate, locomotor activity, feeding behaviour and reproductive output. We found that despite the insecticide dosage being sublethal, both chemical exposure and inbreeding imposed severe developmental costs, reducing the adult emergence rate with no significant interaction effects. However, among the adults, we observed trait-specific phenotypic plasticity. Male locomotor activity was reduced by both stressors, while female activity remained stable. Notably, herbivory significantly increased under both inbreeding and sublethal insecticide exposure. Furthermore, while inbreeding severely reduced both egg production and hatching success, sublethal exposure to the insecticide surprisingly increased hatching success in surviving pairs. These results demonstrate that sublethal insecticide exposure and inbreeding can create a strong developmental bottleneck. Yet, surviving individuals can exhibit compensatory plasticity that maintains or even enhances their capacity for crop damage.