DOI: 10.5061/dryad.nk98sf7vn
title: Pollinator visitation to Na-enriched plants

description: 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.
description: 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 <em>Achillea
 millefolium</em>, <em>Solidago canadensis</em>,
 and<em> Asclepias syriaca</em>. Common pollinators at the site
 include <em>Bombus impatiens</em>, <em>Bombus
 vagans</em>, and <em>Phyciodes tharos</em>.
 We conducted our field experiment using five common plant species
 in meadows of New England: <em>Achillea millefolium</em>
 (Asteraceae), <em>Echinacea purpurea</em> (Asteraceae),
 <em>Geranium sanguineum</em> (Geraniaceae), <em>Monarda
 didyma</em> (Lamiaceae), and <em>Penstemon
 digitalis</em> (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 &gt;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.