Pesticide concentrations in soil, plant tissue, and grasshoppers collected in wildlife refuges in the San Joaquin Valley, California 2024-2025

Pesticide exposure is posited as one of the drivers of arthropod declines, which can in turn affect food availability and safety for vertebrates that consume insects including state and federally endangered species, such as the blunt nosed-leopard lizard (BNLL) (Gambelia sila). State wildlife areas and national wildlife refuges are designed to support species and their habitats; however, many protected areas are embedded in landscapes where pesticides are applied, which could expose species and their prey to pesticide drift. Soil, plant tissue, and grasshopper samples were collected in three wildlife areas in the southern San Joaquin Valley in 2024 (April to October) and 2025 (May to July). Soil and plant tissue samples were collected at four distances from a reserve edge at each site that was chosen close to the neighboring agricultural fields. Grasshoppers were collected when present and recorded whether they were closer to the edge or interior of the site. The samples were then analyzed at U.S. Geological Survey Organic Chemistry Research Lab for 167 pesticide compounds including parent and transformation products. A total of 32 pesticides were detected across all three matrices, with the highest concentrations being that of insecticides and insecticide transformation products. In soil, 20 pesticides were found with the highest concentration being 104.1 nanograms per gram dry weight (ng/g d.w.) of p,p'-DDE and the second highest being 10.6 ng/g d.w. of methoxyfenozide. In plant tissue, 17 pesticides were identified, methoxyfenozide concentration of 585.9 ng/g d.w. was the highest and chlorantraniliprole measured at 215.3 ng/g d.w. second highest. Among insect samples, 6 pesticides were detected, with p,p'-DDE having the highest concentration at 24.8 ng/g d.w. and methoxyfenozide following at 10.7 ng/g d.w.