Pollen-Derived Fatty Acids and Amino Acids Mediate Variance in Pollinator Visitation

Pollinators help maintain functional landscapes and are sensitive to floral nutritional quality. Both proteins and lipids influence pollinator foraging, but the role of individual biochemical components in pollen remains unclear. We conducted an experiment comprising common garden plots of six plant species (Asteraceae, Rosaceae, Onagraceae, Boraginaceae, and Plantaginaceae). These plots were treated with low concentrations of agrochemicals, including fertilizer, herbicide, and a combination of both to induce intra-specific variation in floral chemistry. We recorded insect visitation to inflorescences over two years and eight sites in Dublin, Ireland. We analyzed the pollen amino acid and fatty acid content, quantifying the concentrations of 51 fatty acids and 17 amino acids of the six focal plant species across the four agrochemical treatments. We tested relationships between the pollen composition and the insect visitation matrix as well as an insect trait matrix including sociality, body size, nesting behavior, and whether the insect was a bee or hoverfly. We found: (i) the agrochemical treatments did not affect the biochemical composition of the pollen; (ii) there were many strong associations between fatty acids, amino acids, insect traits, and visitation; and (iii) specific compounds with strong associations (montanic acid, cysteine, and proline) explained more of the variance in insect abundance (honeybees, bumble bees, and hoverflies) than the total amino acid or fatty acid concentrations in the pollen. Our results suggest it is important to evaluate the contribution of individual biochemical compounds in pollen to insect visitation, and also that different insect species respond to different pollen compounds.