Acute toxicity of neonicotinoids and some insecticides to first instar nymphs of a non-target damselfly, Ischnura senegalensis (Odonata: Coenagrionidae), in Japanese paddy fields

Abstract

Systemic insecticides such as neonicotinoids and fipronil are widely applied in rice production. These insecticides have been suspected of reducing biodiversity in paddy ecosystems and reducing wild dragonfly populations in Japan. Conventional ecotoxicological risk assessment could not confirm this, as it has not considered interspecific variation in sensitivity to insecticides. We estimated the median effect concentration (EC50) of 15 systemic insecticides to first instar nymphs of a Japanese damselfly, Ischnura senegalensis (Rambur) (Odonata: Coenagrionidae), commonly found in rice paddy fields. Damselflies were found to be highly sensitive to pyrethroid pesticides, less so to phenylpyrazole, organophosphates, and carbamates, and least sensitive to neonicotinoids, nereistoxin, and diamide. Given the acute toxicity data, the sensitivity of the damselfly to neonicotinoids was considered to be lower than that of other aquatic insects, whereas the EC50 values of the damselfly were 2–3 orders lower than that of Daphnia magna Straus (Diplostraca: Daphniidae), which is a standard test species. These results indicate that the conventional ecological risk assessment based on acute toxicity data of D. magna would underestimate the impact of neonicotinoids on Odonata diversity in paddy ecosystems. We therefore recommend using the paddy-dwelling damselfly as a new test species for insecticide bioassay.