Low exposure concentration effects of methoprene on endocrine-regulated processes in the crustacean Daphnia magna

Methoprene is a growth-regulating insecticide that manifests its toxicity to target organisms by acting as a juvenile hormone agonist. Methoprene similarly may exert toxicity to crustaceans by mimicking or interfering with methyl farnesoate, a crustacean juvenoid. We hypothesized that methoprene interferes with endocrine-regulated processes in crustaceans by several mechanisms involving agonism or antagonism of juvenoid receptor complexes. In the present study, we evaluated this hypothesis, in part, by characterizing and comparing the concentration-response curves for methoprene and several endpoints related to development and reproduction of the crustacean Daphnia magna. Our results demonstrate that methoprene has multiple mechanisms of toxicity and low-exposure concentration effects. Methoprene reduced the growth rate of daphnids with evidence of only a single concentration-response line, having a threshold of 12.6 nM. Molt frequency was reduced by methoprene in a concentration-dependent manner, with a response curve corresponding to a 2-segmented line and thresholds at 4.2 and 0.21 nM. An endpoint related to reproductive maturation, the time of first brood deposition, was also affected by methoprene, with a clear concentration-dependent response and a NOEC of 32 nM. Methoprene reduced fecundity according to a 2-segmented line, with thresholds of 24 and less than or equal to 0.18 nM. These results demonstrate that methoprene elicits significant toxicity to endocrine-related processes in the 5-50 nM concentration range. Furthermore, molting and reproduction were impacted at significantly lower methoprene concentrations, with a distinct concentration response and a threshold of less than or equal to0.2 nM. The different concentration-dependent response from that of methoprene could involve agonism or antagonism of various juvenoid receptor configurations.