Effects of imidacloprid on Afrotropical aquatic ecosystems: a South African microcosm study

Pesticides are widely used to meet the food demands of a growing population, with various types used to control pests depending on the crops grown. Rainfall, overspray, and runoff from agricultural fields can wash these insecticides into water bodies, posing documented environmental risks. Imidacloprid is commonly used in Afrotropical regions such as South Africa, yet limited information is available on its toxicity to aquatic ecosystems within this climate region. To address this, a 16-week outdoor microcosm study was conducted, exposing aquatic communities to 0.1, 10, and 250 µg/L of imidacloprid under natural conditions. Physicochemical parameters, such as pH, dissolved oxygen, temperature, and various others, were monitored regularly. The microbial community, organic decomposition, and zooplankton were largely unaffected by the exposure to imidacloprid. In contrast, 10 and 250 µg/L of imidacloprid led to long-term effects on the macroinvertebrate community, while short-term effects were found in the 0.1-µg/L treatment. Families such as Coenagrionidae were affected across all treatments, while Caenidae and Notonectidae were affected in only the highest treatment. Changes in the community structure resulted in reduced predator abundance (e.g., Coenagrionidae, Libellulidae, and Notonectidae) and increased filter feeders (Culicidae). High temperatures likely increased the toxicity to arthropods when compared with similar studies with lower light intensities and temperatures. Recovery was limited during the postexposure period, with some families (Baetidae) rebounding quickly, while others (Caenidae and Coenagrionidae) showed delayed recovery. Full recovery of the macroinvertebrate community systems did not occur during the eight-week recovery period, highlighting the importance of unexposed refuges for recolonization. This study provides important field-realistic evidence of the ecological risks associated with imidacloprid use in Southern African freshwater systems.