Polystyrene microplastic pollution: An abiotic factor affecting the defense of Ulmus pumila and biocontrol of Bacillus thuringiensis against Hyphantria cunea

Soil pollution is a key environmental factor shaping the occurrence of phytophagous pest outbreaks. This study investigated the defense responses of Ulmus pumila and the biocontrol performance of Bacillus thuringiensis (Bt) against Hyphantria cunea larvae within a food-chain context involving polystyrene microplastics (PS)-contaminated soil - U. pumila - H. cunea larvae. Larvae consuming PS-exposed U. pumila leaves exhibited significant declines in body weight, body length, head capsule width, and nutrient content, accompanied by disrupted expression of growth-regulatory genes. PS exposure induced substantial increases in secondary metabolite levels and upregulated key genes associated with flavonoid biosynthesis, whereas it reduced primary metabolite contents and photosynthetic pigment levels, along with downregulating genes involved in sugar and amino acid metabolism in leaves. PS further intensified larval mortality through a synergistic effect and enhanced the growth-inhibitory activity of Bt. Relative to Bt alone, combined PS and Bt treatment resulted in pronounced downregulation of energy metabolism pathway genes and nutrient content, elevated transcription of midgut apoptosis-related genes, impaired cellular immune function, and irregular humoral immune responses. Overall, soil PS contamination functions as an abiotic stressor influencing the defensive capacity of U. pumila and modulating the biocontrol efficacy of Bt against H. cunea.