Physiological and histopathological effects of polystyrene and polyethylene terephthalate microplastics and their binary mixture in a ground beetle ( Blaps polychresta )

Abstract

Microplastic (MP) contamination of soils is increasing, yet mechanistic evidence for mixture toxicity in terrestrial insects remains limited. We investigated the sub‐chronic (30‐day) dietary effects of polystyrene (PS‐MP), polyethylene terephthalate (PET‐MP) and their binary mixture (1% PS + 1% PET; total 2% w/w) on the ground beetle Blaps polychresta . Endpoints included defecation rate, digestive enzyme activities (amylase and protease), oxidative stress biomarkers (reactive oxygen species [ROS], catalase [CAT] and superoxide dismutase [SOD]) and histopathological alterations in the midgut and testis. Survival was unaffected; however, PS and PET individually reduced defecation rate, whereas the mixture did not further exacerbate gut motility impairment. All treatments significantly altered digestive physiology, with increased amylase and reduced protease activity and the strongest imbalance occurring in the mixture group. ROS levels were elevated in both midgut and testicular tissues, accompanied by significant upregulation of CAT and SOD, indicating oxidative stress–mediated responses. Histological investigation revealed epithelial disruption and muscle disorganization in the midgut and degeneration of testicular follicles, vacuolization, pyknosis of spermatocytes and reduced follicular integrity. Trypan blue staining confirmed increased cellular damage in the testes following MP exposure. Testicular follicle diameter decreased in PS‐ and PET‐exposed beetles, indicating impaired spermatogenesis. Overall, chronic ingestion of PS and PET disrupts digestive and male reproductive physiology through oxidative stress–linked mechanisms, with mixture exposure intensifying several biochemical and histopathological effects. These findings highlight the importance of incorporating polymer interactions into terrestrial insect risk assessments.