Tallgrass prairie wildlife exposure to spray drift from commonly used soybean insecticides in Midwestern USA

Insecticides are widely used in the Midwestern USA to combat soybean aphids (Aphis glycines), a globally important crop pest. Broad-spectrum foliar insecticides such as chlorpyrifos, lambda-cyhalothrin, and bifenthrin (hereafter, “target insecticides”) are toxic to wildlife in laboratory settings; however, little information exists regarding drift and deposition of these insecticides in fragmented tallgrass prairie grasslands such as those in Minnesota, USA. To address this information gap, target insecticide spray drift and deposition were measured on passive samplers and arthropods in grasslands adjacent to crop fields in Minnesota. Samples were collected at focal soybean field sites immediately following target insecticide application and at reference corn field sites without target insecticide application. Target insecticides were detected 400 m into grasslands at both focal and reference sites. Residues of chlorpyrifos, an insecticide especially toxic to pollinators and birds, were measured above the contact lethal dose (LD₅₀) for honey bees (Apis mellifera) up to 25 m from field edges in adjacent grasslands. Chlorpyrifos residues on arthropods were below the acute oral LD₅₀ for several common farmland bird species but were above the level shown to impair migratory orientation in white-crowed sparrows (Zonotrichia leucophrys). Deposition of target insecticides on passive samplers was inversely associated with distance from field edge and percent canopy cover of grassland vegetation, and positively associated with samplers placed at mid-canopy compared to ground level. Target insecticide deposition on arthropods had an inverse relationship with vertical vegetation density and was positively associated with maximum height of vegetation. Tallgrass prairie with cover ≥25 m from row crop edges may provide wildlife habitat with lower exposure to foliar application insecticides. Prairie management regimes that increase percent canopy cover and density of vegetation may also reduce exposure of wildlife to these insecticides.