Exposure pattern-specific species sensitivity distributions for the ecological risk assessments of insecticides

In the higher tiers of pesticide risk assessment, the Species Sensitivity Distribution (SSD) concept is often used to establish the effect threshold defined as the concentration protecting 95% of the species (Hazardous Concentration 5%, HC5). The toxicity data included in SSDs are normally established using a constant exposure regime. However, the exposure of pesticides in the field is often characterised by a variable exposure regime. Toxicokinetic-toxicodynamic (TKTD) models can be used to extrapolate the toxic effects of a chemical to a specific, time-variable exposure regime. The aim of this paper was to develop Exposure Pattern Specific SSDs (EPS-SSDs) for three insecticides using TKTD models and to compare the HC5 of different exposure patterns with the same time-weighted average concentration to evaluate whether the use of EPS-SSDs would change the outcome of the ecological risk assessment. The EPS-SSDs were developed by estimating TKTD parameters for the compounds chlorpyrifos, imidacloprid and lambda-cyhalothrin using results from standard, 96 h, single species tests. These parameter estimates were used for TKTD modelling to determine toxicity thresholds (e.g. LC10 and LC50) for contrasting exposure patterns after certain evaluation times (4, 10 or 100 days). HC5 values were constructed with TKTD-predicted LC10- and LC50- values for different exposure patterns characterised by similar time-weighted average concentrations. Differences between those HC5 values ranged from a factor 1 to a factor 2.3 for the short evaluation period (4 d). This difference was smaller when using an evaluation period of 10 days instead of 4 days and selecting the TKTD-predicted LC10 instead of TKTD-predicted LC50 based HC5s. For the long term evaluation period (100 d), a maximum difference of a factor of 30 was found.