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

Flower-visiting insects in agroecosystems forage on weeds exposed to
agrochemicals that may compromise the quality of their floral resources.
We conducted complementary field and greenhouse experiments to evaluate:
1) the effect of low concentrations of agrochemical exposure on nectar and
pollen quality and 2) the relationship between floral resource quality and
insect visitation. We found pollen amino acid concentrations were lower in
plants exposed to low concentrations of herbicide, and pollen fatty acid
concentrations were lower in plants exposed to low concentrations of
fertilizer, while nectar amino acids were higher in plants exposed to low
concentrations of either fertilizer or herbicide. Exposure to low
fertilizer concentrations also increased the quantity of pollen and nectar
produced per flower. The responses of plants exposed to the experimental
treatments in the greenhouse helped explain insect visitation in the field
study. The insect visitation rate correlated with nectar amino acids,
pollen amino acids, and pollen fatty acids. An interaction between pollen
protein and floral display suggested pollen amino acid concentrations
drove insect preference among plant species when floral display sizes were
large. We show that floral resource quality is sensitive to agrochemical
exposure and that flower-visiting insects are sensitive to variation in
floral resource quality.