Exposure assessment for terrestrial non-target arthropods

Concerns regarding the potential of pesticides to harm terrestrial non-targ et arthropod populations have led to the in creased use of ecotoxicological test systems for terrestrial Non-target Arthropod risk assessment. Whilst some useful guidance on terrestrial invertebrate test systems is available, there are significant gaps in guidance for terrestrial non-target arthropo d exposure estimates. The typical exposure in the standard test systems is by application of the test substance at the field rate (i.e. gram substance per hectare field) on to a two dimensional surface. However, under field c onditions such a spray deposit will be diluted over the total available 3-D imensional plant and soil surface. The recommendation is to use published l eaf area index and crop interception values to standardize terrestrial dilu tion factors, which can then be used to predict exposure on a 3-Dimensional plant surface. Based on average crop/time specific LAI data for 26 crops, a surrogate off-crop dilution factor of 12 was calculated which can be used to convert 2-Dimensional spray drift exposure to 3-Dimensional off-crop pl ant surface exposure. Another significant terrestrial exposure guidance gap is how to calculate predicted environmental concentrations (PECs) for mult iple application products. Based on spray interval and half-life data from 32 representative multiple application plant protection products, the typic al worst-case PECs for accumulation of residues were calculated after up to 8 applications. These data showed that Multiple Application Factors (MAFs = accumulated PEG/initial PEG), increased from 1 to 3.5 after 1 to 8 applic ations, respectively. Finally, overall 90(th) percentile spray deposit valu es have been proposed for deriving off-crop multiple applications PECs (1 t o 8 applications) based on published spray drift data. The recommended equa tions for terrestrial exposure assessment include the use of:application ra te, the 90(th) percentile drift value, the multiple application factors (MA F) and the standard dilution factor (for 3-Dimensional plant surface). This proposed terrestrial non-target arthropod exposure scheme is comparable wi th other first tier exposure assessment schemes eg aquatic exposure assessm ent.