Fertilizer and herbicide alter nectar and pollen quality with consequences for pollinator floral choices

Flower-visiting insects in agroecosystems forage on field-edge weeds often exposed to agrochemicals that may compromise the quality of their floral resources. We conducted complementary field and greenhouse experiments to evaluate the: 1) effect of low concentrations of agrochemical exposure on nectar and pollen quality and 2) relationship between floral resource quality and insect visitation. We found pollen amino acid concentrations were negatively affected by herbicide exposure, and pollen fatty acid concentrations were negatively affected by fertilizer exposure, while nectar sugar concentrations were positively affected by both fertilizer and herbicide exposure. These variations found in the greenhouse helped explain insect visitation in the field study. The insect visitation rate correlated with floral resource quality. An interaction between pollen protein and floral display suggested pollen protein drove insect abundance for larger display sizes. We show that flower-visiting insects can detect and respond to changes in floral resource quality mediated by agrochemicals.