Effects of Microplastics and Cadmium on the Leptinotarsa decemlineata (Coleoptera: Chrysomelidae): An Evaluation Using a Two-Sex Life Table

This study utilized the age-stage, two-sex life table method to evaluate the toxic effects of polyethylene microplastics (PE, 300 mg/kg) and cadmium (Cd, 30 mg/kg), both individually and combined, on Leptinotarsa decemlineata. Compared to controls, all treatments significantly prolonged larval development and reduced survival, lifespan, and fecundity. The combined exposure (PE + Cd) exerted the strongest inhibition: the total pre-adult developmental duration (TPOP) increased by 18.8% (38.00 days), while the intrinsic growth rate (r) dropped by 59.0% to 0.0273 d−1. Additionally, the net reproduction rate (R0) and fecundity fell to their lowest levels (5.08 and 19.06, respectively), significantly lower than in single-treatment groups. Age-stage life expectancy analysis confirmed severe survival pressure in the combined group, evidenced by a 30% reduction in first-instar survival and a 14-day shortened adult lifespan. These findings demonstrate the synergistic toxicity of PE and Cd co-contamination, providing critical data for ecological risk assessment in the “soil–plant–herbivore” system and integrated pest management strategies.