Influence of light in acute toxicity bioassays of imidacloprid and zinc pyrithione to zooplankton crustaceans

The acute toxicity of imidacloprid, a neonicotinoid insecticide, and zinc pyrithione (Zpt), a biocide used in anti-dandruff shampoos and protective antifouling paints, to three species of ostracods and two waterfleas, including Daphnia magna, was determined and compared under light and dark conditions. Under normal laboratory conditions, UV light had no significant influence on the outcome of toxicity bioassays, although in the case of imidacloprid both EC50 and LC50 calculated values were twice as high under the light as in the dark. No influence of UV light was observed on bioassays conducted with Zpt, in spite of the fast aqueous photolysis exhibited by this compound. Imidacloprid 48-h LC50 for cladocerans (65–133 mg/L) were two orders of magnitude higher than for ostracods (301–715 μg/L); values of EC50 for cladocerans and ostracods were 2–6 mg/L and 3–16 μg/L, respectively. Toxicity of Zpt to both ostracod and cladoceran species appears to be similar, with 48-h LC50 in the range 137–524 and 75–197 μg/L for ostracods and cladocerans, respectively, and similar values for EC50s. The mortality endpoint (LC50), however, is not a reliable predictor of the effects of imidacloprid under field situations (e.g. rice paddies), because the paralysis effect induced by this insecticide takes place at much lower concentrations than those required to cause the death of the animals: regardless of the taxa, differences as large as 100- or 600-fold were observed between the EC50 and LC50 for the same exposures. As a consequence, immobilization tests and EC50 values are recommended for this class of compounds, while caution should be exercised in environmental risk assessments of this and possibly other related neonicotinoid insecticides with similar activity.