Insecticide drift and impacts on arthropod prey resources of birds in public grasslands in Minnesota

Abstract

Increasing evidence suggests that exposure to pesticides may be a considerable threat to grassland wildlife in agricultural landscapes. In Minnesota, many remaining grasslands are fragmented and surrounded by row crops, including soybeans that are routinely treated with foliar insecticides to control soybean aphids. These insecticides have been shown to be highly toxic to non-target organisms including birds and pollinators in laboratory settings; however, little information exists regarding the impact of spray drift on free-ranging wildlife. My objectives were to evaluate direct and indirect effects of soybean aphid insecticide applications on grassland birds and their insect food resources in the agricultural region of Minnesota. My treatment study sites were comprised of public grasslands bordering privately-owned soybean fields treated with foliar insecticides and non-treated control sites, and I collected samples during the summers of 2017 and 2018. I quantified chemical deposition to assess the extent of insecticide drift into public grasslands and surveyed the insect community pre- and post-application of insecticides to measure changes in abundance, biomass, and richness of insect food for grassland birds. I detected insecticides, primarily chlorpyrifos, in grasslands up to 400 m from field edges regardless of whether adjacent fields were sprayed with insecticides, and deposition was greatest within 25 m of field edges. I measured chlorpyrifos residues that were above the contact LD50 for honey bees up to 25 m from field edges in grasslands. The masses of chlorpyrifos that birds could consume in a day (if food items contained chlorpyrifos residues equivalent to those in my arthropod samples) were below the acute oral LD50 values for several grassland bird species. I used linear mixed models in a hierarchical selection approach to assess the importance of distance from field edge, spray method (plane or ground sprayer), and sampler height (mid-canopy or ground) in explaining insecticide deposition in grasslands. The best-supported model of deposition on passive sampling devices included an inverse association of distance from the field edge with deposition and positive association of samplers being placed at the mid-canopy level compared to ground level. Canopy cover of live vegetation had an inverse association with deposition. The best-supported model of insecticide deposition on arthropods included effects of air temperature and maximum height of vegetation. Short-term reductions in total arthropod abundance, bird prey abundance, and Coleopteran family richness occurred in grasslands bordered by fields sprayed with foliar insecticides. The total abundance of arthropods in grasslands bordering sprayed soybean fields was lower 3–5 days after insecticide applications. The abundance of arthropods important in grassland bird diets (specifically, Araneaens, Coleopterans, Orthopterans, and Lepidopteran larvae) was also lower after nearby spraying, with lower abundance measured in treatments sites 19–21 days post-spraying. Coleopteran family richness at treatment sites was lower than control sites 3–5 days after insecticide applications. Measures of total consumable dry biomass, bird prey biomass, family richness of Araneaens, family richness of Hemipterans, and family richness of Orthopterans were not different between treatment and control sites post-spraying. My findings suggest that spray drift from soybean aphid insecticides is occurring in grasslands in Minnesota and may be impacting food resources for grassland birds and other insectivorous wildlife. My results can inform land acquisition and management decisions to reduce impacts of insecticide spray drift on grassland wildlife in agricultural landscapes.