Plants have evolved a variety of approaches to attract pollinators, including enriching their nectar with essential nutrients. Because sodium is an essential nutrient for pollinators, and sodium concentration in nectar can vary both within and among species, we explored whether experimentally enriching floral nectar with sodium in five plant species would influence pollinator visitation and diversity. We found that the number of visits by pollinators increased on plants with sodium-enriched nectar, regardless of plant species, relative to plants receiving control nectar. Similarly, the number of species visiting plants with sodium-enriched nectar was twice that of controls. Our findings suggest that sodium in floral nectar may play an important but unappreciated role in the ecology and evolution of plant-pollinator mutualisms.

We conducted this work in a meadow at the University of Vermont’s Aiken Forestry Sciences Laboratory in South Burlington, Vermont (elevation: 60m; 44°27'09.0"N 73°11'26.7"W). Common flowering species at the site include Achillea millefolium, Solidago canadensis, and Asclepias syriaca. Common pollinators at the site include Bombus impatiens, Bombus vagans, and Phyciodes tharos.

We conducted our field experiment using five common plant species in meadows of New England: Achillea millefolium (Asteraceae), Echinacea purpurea (Asteraceae), Geranium sanguineum (Geraniaceae), Monarda didyma (Lamiaceae), and Penstemon digitalis (Plantaginaceae). We obtained these plants from two local greenhouses early in the growing season and maintained at least 12 individuals of each species in 23-cm diameter pots in a greenhouse at the site. Plants were kept in a greenhouse, watered twice a day, and were not treated with fertilizer.

We experimentally manipulated Na content in half of the plants to examine the influence of Na-enriched nectar on plant-pollinator interactions. We used two stock solutions of artificial nectar: control nectar which was made up of 35% sucrose weight:volume and Na-enriched nectar which was made up of 35% sucrose + 1% Na weight:volume. We applied 15 μL of our artificial nectar thirty minutes before each pollinator observation period (at approximately 0730, 1130, and 1530) by inserting the tip of a micro-pipette between the ovary and the stamens of each flower. Plants were in bloom for the duration of the experiment. Pipette tips were changed between each plant to avoid accidental cross-pollination or accidental application of Na to control flowers.

Prior to our pollinator observations each day (see details below), we selected 6 individual plants from each of our focal species, half of which received Na-enriched nectar and the other half received control nectar. We applied artificial nectar treatments randomly to individual plants. We then randomly placed the plants (in their pots) along six 15-m transects. Transects were approximately three meters from one another. Plants that were designated as Na-enriched received the experimental solution exclusively for the duration of the experiment.

We recorded pollinator visits to flowers from July 9 to August 2, 2019 on warm, sunny days (no rainfall, ambient temperature >8°C) during three one-hour observation periods beginning at 0800, 1200, and 1600. We recorded the identity and number of each pollinator visiting the flowers of each species across all 30 plants. After each observation period, we walked each of the six 15m transects for 10 minutes each, and netted visitors on the focal plant species to estimate diversity of floral visitors. If we were unable to be identify a floral visitor in the field, we collected it and stored it in 75% EtOH. We identified each pollinator to species or the finest taxonomic level possible.

To ascertain whether Na-enriched plants were visited more frequently by pollinators, we calculated the total number of visitors across all observation periods and days for each individual plant for each species in each treatment. Each individual plant in each treatment was observed, on average, 24 times over the course of the experiment, and each focal plant species was observed, on average, 270 times over the course of the experiment.