Differences in susceptibility of five cladoceran species to two systemic insecticides, imidacloprid and fipronil

The authors determined the 48-hour semi-static acute toxicity (immobility endpoint) of imidacloprid to five cladoceran species, and then compared those species sensitivity distributions to those from all other tested aquatic species in the ECOTOX database. The test species were female neonates of Ceriodaphnia dubia, C. reticulata, Daphnia magna, D. pulex, and Moina macrocopa. Imidacloprid concentrations were derived from the formulated product Admire (nominal test concentrations depended on the species, but ranged from 390.63 to 400,000 µg/L). Since the test was semi-static due to the rapid photolysis of imidacloprid, the results may be slightly overestimated compared to static 48 hour tests. The imidacloprid EC50s were: C. dubia - 571.62 µg/L; C. reticulata - 5,552.9 µg/L; D. magna - 43,265 µg/L; D. pulex - 36,872 µg/L; and M. macrocopa - 45,271 µg/L. There were clear differences in susceptibility among the species tested, with the most sensitive being about 100 times more sensitive than the least. When compared to other aquatic data from ECOTOX, the tested cladocerans were significantly less sensitive than the other species pooled, with cladocerans about 500 times less sensitive than most other aquatic taxa. The authors suggest that Ceriodaphnia spp. may be a more appropriate indicator of imidacloprid effects than D. magna since it is more sensitive, given the wide ranging sensitivities.