Impact of microplastics on the health and reproductive success of aquatic insects

While the world’s attention has focused on plastic debris drifting through ocean gyres, a quieter contamination crisis is unfolding in rivers, wetlands, and streams where aquatic insects spend their most vulnerable life stages. This research assessed the effects of microplastic pollution on the health and reproductive output of three dominant aquatic insect orders Ephemeroptera (mayflies), Trichoptera (caddisflies), and Odonata (dragonflies) across 18 freshwater sites in the Western Cape Province of South Africa. Sediment and water samples were collected quarterly from September 2023 to December 2024, and microplastic concentrations were quantified by polymer type and size class. Concurrently, insect reproductive success was estimated through egg-mass counts, larval survival assays, and adult emergence rates. Microplastic densities ranged from 0.3 to 18.2 particles per gram of dry sediment, with polyethylene (31.4%) and polypropylene (22.8%) dominating the polymer profile. Scatter plot analysis revealed significant negative relationships between microplastic concentration and reproductive success for all three orders: mayflies showed the steepest decline (R² = 0.67, slope = −2.8% per particle/g), followed by caddisflies (R² = 0.58) and dragonflies (R² = 0.49). At sites exceeding 10 particles per gram, mayfly egg production dropped by 47.3% and larval survival fell by 38.6% relative to the least contaminated reference sites. The flowchart developed from these data traces the impact pathway from microplastic sources through sediment accumulation, ingestion, physiological damage, reproductive impairment, and population-level decline. These findings suggest that microplastic contamination in South African freshwaters has already reached levels capable of suppressing aquatic insect reproduction, with downstream consequences for the fish, birds, and ecosystem services that depend on healthy insect populations.