Seasonal sensitivity of Gammarus pulex towards the pyrethroid cypermethrin

The aquatic toxicity of insecticides like the pyrethroids have been discussed intensively over the recent years especially in relation to risk assessment and how seasonality may or may not affect the sensitivity of non-target organisms. To address this issue, the crustacean Gammarus pulex was collected once a month for 16 months and acclimated to 10 °C for four days before being exposed to a 90 min pulse of cypermethrin. In vitro cytochrome P450 activity, total lipid content, total protein content, and dry weight were measured in male and female gammarids from each sampling date and used along with the water temperature as variables for sensitivity prediction by Partial Least Squares (PLS) regression models. The 24 h EC₅₀-values varied more than 30 fold across the sampling period from 0.21 ± 0.05 μg L^-1 (April 2015) to 6.60 ± 3.46 μg L^-1 (October 2015), indicating seasonal variances in the acute sensitivity of G. pulex towards cypermethrin. After 168 h of recovery this difference in EC₅₀-values was reduced to seven-fold. In both male and female gammarids seasonal patterns were observed in the total lipid content and in vitro CYP P450 activity, which peaked in spring and fall, respectively. The current study shows the importance of reporting time of organism collection and experimental execution for risk assessment of pyrethroids as season is important for the acute sensitivity of G. pulex. We suggest prolonged acclimation times of sampled macroinvertebrates to constant laboratory conditions in order to even out possible seasonal differences in sensitivity.