Date of Award


Document Type


Degree Name

Master of Science (MS)



First Advisor

Sara Helms Cahan

Second Advisor

Alison Brody


Pesticides are widely used in agriculture to minimize the negative impact of harmful pests. However, the use of pesticides also has non-target effects on beneficial insects such pollinators. In addition to the direct harm to pollinators foraging on crops during pesticide application, pesticides are also frequently utilized in non-agricultural settings, and can drift to non-target areas via the wind and water. Plants growing in such contaminated soils may absorb pesticides and express them in their flowers, specifically the pollen and nectar upon which pollinators rely. This is particularly important because encouraging wildflower growth alongside fields is a common strategy to provide additional resources to pollinators, which may pose an unrecognized additional risk to foraging bees. The aim of my research was to 1) determine the extent to which non-agricultural areas harbor harmful pesticides that pose a risk to bees, and 2) investigate the risk posed by wildflowers within agricultural areas that can be contaminated via pesticide drift.To investigate pesticide occurrence in non-agricultural areas, I measured pesticide residues in seven non-agricultural and two agricultural sites in the Champlain Valley of Vermont in the summer of 2019. Soil, floral, and bee samples were collected and analyzed for pesticide residues. Pesticide residues found in non-agricultural areas were mainly found at concentrations below levels of concern, indicating these areas do not pose a significant risk to pollinators. Higher risk values were mainly observed within one of the agricultural sites, likely driven by higher pesticide use associated with fruit orchards. To assess the risk posed by wildflower refuges growing within conventional agricultural settings, I combined field observations with a pesticide expression study to build a risk model for foraging bees. I assessed floral availability and visitation by both managed honeybees and wild bees in nine belt transects of unmown areas adjacent to agricultural fields at the University of Vermont Horticultural Research and Education Center in South Burlington, Vermont. Unmown areas contained a total of 37 wildflower species, with abundant floral resources growing throughout the season. Bees foraged within these unmown areas, with wild bee visitation consistently seen throughout the summer. Honeybee visitation peaked strongly in August. For two of the species that were most visited by bees, red clover (Trifolium pratense) and English plantain (Plantago lanceolata), I quantified the extent to which they expressed soil-derived pesticides by growing them from seed in a greenhouse and exposing them to 0.193 mg of Imidacloprid and/or Difenoconazole. Imidacloprid was expressed at a rate of 0.66 and 0.46 for English plantain and red clover, respectively. Difenoconazole was expressed at a rate of 0.64 and 0.10 for English plantain and red clover, respectively. These results were used to construct a predictive model of exposure risk as a function of soil contamination and bee foraging behavior, and risk assessment values based on honeybees. The model shows that bees could be exposed to up to 15% or 11% of their lethal dose when foraging on English plantain or red clover, respectively, in one day. When foraging in the contaminated landscape for just 30 % of the time, or in areas with as little as 25 ppb of one pesticide, bees are exposed to 3% of their lethal dose, the minimum level of concern. This study shows wildflowers in agriculture do pose a risk to bees, given their proximity to agricultural pesticide use and potential to express pesticides. If possible, alternative methods of pollinator aid should be incorporated within these areas, and wildflower plantings should incorporate buffer zones to minimize contamination.



Number of Pages

107 p.

Available for download on Saturday, October 21, 2023