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

Authors: GOEBEL, KATELIN - University Of Minnesota; DAVROS, NICOLE - Minnesota Department Of Natural Resources; ANDERSEN, DAVID - Us Geological Survey (USGS); Rice, Pamela

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/13/2021
Publication Date: 4/20/2022
Citation: Goebel, K.M., Davros, N.M., Andersen, D.E., Rice, P.J. 2022. Tallgrass prairie wildlife exposure to spray drift from commonly used soybean insecticides in Midwestern USA. Science of the Total Environment. 818. Article 151745. https://doi.org/10.1016/j.scitotenv.2021.151745.
DOI: https://doi.org/10.1016/j.scitotenv.2021.151745

Interpretive Summary: Insecticides are widely used in farmland regions in the Midwestern USA to combat soybean aphids, a globally important crop pest first discovered here in 2000. Commonly used broad-spectrum foliar insecticides have been shown to be toxic to wildlife in laboratory settings; however, little information exists regarding drift and deposition of insecticides in fragmented grasslands in agricultural landscapes such as those found in Minnesota, USA. To address this information gap, insecticide drift and deposition were measured on passive samplers and arthropods in grasslands adjacent to soybean fields in western Minnesota. Samples were collected immediately following insecticide application at treatment sites and at control sites without insecticide application. Insecticides were detected in grasslands up to 400 m from field edges regardless of whether adjacent fields were sprayed with insecticides. Residues of chlorpyrifos, an insecticide especially toxic to wildlife, were measured above the LD50 (a contact dose that is lethal to 50%) for honey bees up to 25 m from field edges in adjacent grasslands. Calculations to evaluate the quantity of chlorpyrifos birds could consume in a day when eating arthropods with the measured chlorpyrifos residues were below the oral dose that would be lethal to 50% of the population (acute oral LD50). Analysis of the data showed residues of the insecticide on the passive samplers decreased with an increase in distance from the field edge and greater quantities were found on samplers placed at the mid-canopy level compared to the ground level. The greater percentage of canopy cover the less chemical residue was measured on arthropods, which was also influenced by air temperature and maximum height of vegetation. Knowledge gained from this research can be used by land managers to develop strategies to protect wildlife. For example, grasslands with cover greater than 25 m from row crop edges may provide wildlife habitat with lower exposure to foliar application insecticides. Management regimes that increase the percentage of canopy cover in grasslands also have the potential to reduce exposure of grassland wildlife to these insecticides.

Technical Abstract: 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.