Ana‐Belén Muñiz‐González

Pesticides employed worldwide for crop protection easily reach aquatic systems, which act as the main reservoirs, and become a risk factor for aquatic fauna. Fipronil is a broad‐spectrum insecticide acting on the insect nervous system; however, other effects and systems unrelated to this mechanism could be affected in non‐target organisms. Thus, the present study aimed to assess the impact of fipronil on the suborganismal response (gene expression and enzymatic activity) of Chironomus riparius larvae as a model organism in ecotoxicology. To this end, short‐term toxicity tests were carried out with fourth‐instar larvae exposed to 0.001, 0.01, and 0.1 µg L −1 of fipronil for 24 and 96 h. Messenger RNA levels of 42 genes related to diverse metabolic pathways were analyzed by real‐time polymerase chain reaction, complemented with catalase (CAT), glutathione S ‐transferase (GST), and acetylcholinesterase (AChE) activities. Few effects were observed at 24 h; however, after longer exposure (96 h), genes involved in the endocrine, detoxification, stress, and immune response pathways were altered. Moreover, fipronil at 96 h increased CAT and GST activity at 0.01 µg L −1 and AChE at the highest concentrations. The results demonstrate that even low environmentally relevant fipronil concentrations can modulate the molecular response of several cellular pathways in C. riparius after short‐term exposure. These results bring new information about the underlying response of fipronil and its mode of action on a key aquatic invertebrate. Despite no effects on mortality, strong modulation at the suborganismal level emphasizes the advantage of biomarkers as early damage responses and the harmful impact of this pesticide on freshwater organisms. Environ Toxicol Chem 2024;43:405–417. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.