Effects of realistic pesticide mixtures on wheat ( <i>Triticum aestivum</i> ) and lettuce ( <i>Lactuca sativa</i> )

The application of pesticides often results in exposure and risks to non-target organisms. Among these organisms are non-target terrestrial plants (NTTPs), which are essential for maintaining healthy ecosystems. Despite the rigorous risk assessment process for approving pesticides used in Europe and many developed countries, the impacts of environmentally realistic mixtures remain insufficiently understood. This study evaluated the effects of 11 realistic mixtures on two plant species. Each mixture comprised 5 pesticides selected based on their estimated risk to terrestrial plants (from regulatory reports) and their frequency of detection in agricultural soils across 10 European countries and Argentina. Two standardized assays, root growth inhibition test (International Organization for Standardization [ISO] 11269-1) and seedling emergence and growth test (Organisation for Economic Co-operation and Development, test guideline 208), were conducted with wheat (Triticum aestivum) and lettuce (Lactuca sativa). Three concentrations were used: median measured environmental concentration (MEC), predicted environmental concentration (PEC), and five times PEC (5PEC). Significant effects were observed for ten of the eleven mixtures, and these were species-specific and endpoint-specific, likely reflecting variation in mixture composition and herbicide modes of action. Root growth was more sensitive than seedling emergence and shoot biomass. When considering both MEC and PEC treatments, root growth was affected in 9 mixtures for both species, while emergence and biomass were affected in 5 and 7 mixtures, respectively. These results indicate that realistic combinations of pesticide residues can cause deleterious effects on terrestrial plants even at concentrations below predicted environmental thresholds, where no adverse impact would ordinarily be expected. Given that current European risk assessments emphasize spray drift and single-compound exposures, consideration of mixture effects is crucial for adequately evaluating risks to NTTPs.