Indirect effects of insecticide exposure under warming conditions considering predator–prey interactions in a freshwater ecosystem

Understanding the ecological impact of insecticides on freshwater ecosystems, including paddies, through biotic interactions is essential for effective ecosystem management. However, predicting these ecological effects of insecticides could become more difficult in the future, as insecticide toxicity may increase under warming conditions. For example, species that are not normally affected by insecticides could be affected; therefore, the indirect effects of pesticides may be more complicated. This study used simple microcosms to predict the pathway and strength of the combined effects of the insecticide fipronil (widely used in Japanese paddy fields) and increased water temperature (approximately 4 °C higher than ambient conditions: IPCC’s worst-case scenario) on paddy ecosystems via predator–prey interactions. We focused on the interactions among dragonfly nymphs (predator), snails (prey), and producers (dissolved oxygen was monitored as an indicator of primary producers consumed by snails) in a paddy ecosystem. Fipronil had a positive indirect effect on the snails through a reduction in dragonflies under ambient water temperature conditions. Under warming treatments, the fipronil also affected dissolved oxygen, which also indirectly benefitted snails. The positive indirect effect of fipronil on snails increased with increasing water temperature and resulted in an extreme increase in the number of individuals. Given these results, in ecological risk assessments of warming scenarios, it is essential not only to obtain toxicity data for various chemicals but also to predict the processes by which chemicals spill over into ecosystems via biotic interactions.