University of Michigan Professor of Ecology and Evolutionary Biology Nathan Sanders and his colleagues set out to test whether pollinators have a taste for salt in flower nectar.

Their fascinating results were published March 2, 2022 in Biology Letters. Researchers have done similar investigations with one or two species, but this is the first time it's been tested across multiple species in a natural environment. 

The researchers selected five species of flowering plants native to the meadows of Vermont, the site of the research. They grew the flowers, including yarrow and purple coneflower, in a greenhouse. The lead author, Carrie Finkelstein, who was an undergraduate at the University of Vermont at the time, replaced half of the flowers’ nectar with a sugary and non-salty solution and  the other half with a sugary solution containing one percent salt.

Sodium helps animals balance the amount of water in their cells and plays an important role in muscle function. It’s difficult for plant eating creatures to get enough salt from their diets. “Herbivores have to get salt from somewhere,” said Sanders. “The same is true for things that fly around pollinating plants.”

The research team found that the number of visits by pollinators nearly doubled on plants with sodium-enriched nectar, regardless of plant species, relative to plants receiving control nectar. Likewise, sodium-enriched nectar attracted more diverse visitors with about twice as many species of pollinators stopping by. Bees visited the most, including Western honeybees (Apis mellifera) and bumble bees, with flies and butterflies following.

The findings suggest that sodium can draw pollinating insects to plants. Climate change is expected to alter the water cycle and the availability of sodium to plants, which might have negative effects on plant populations and on visitors to their flowers, the authors wrote. Furthermore, their findings suggest that sodium in floral nectar may play an important but unappreciated role in the ecology and evolution of plant–pollinator mutualisms.

As far as advice for farmers or gardeners, “Farmers put out salt licks for the cattle, but it’s probably not a good idea to to put out salt licks for pollinators. The pollinators might be drawn to the salt lick instead of the plants.” 

“The next important steps are to understand what that extra sodium does for the pollinators - does it help them and by how much?” Sanders added. Ethan VanValkenburg, an EEB U-M undergraduate, is going to the Rocky Mountain Biological Lab this summer to work with Paul CaraDonna and Sanders on how salty nectar influences plant-pollinator interactions (focusing on bumble bees) in the Rocky Mountains. Sanders and his associates may do some experiments at the E.S. George Reserve and U-M Biological Station this summer – so stay tuned for the latest buzz.

Sanders coauthors include Paul J. CaraDonna and Andrea Gruver, Chicago Botanic Garden; Carrie J. Finkelstein, Environmental Program, Rubenstein School of Environment and Natural Resources, University of Vermont; Ellen A. R. Welti, Conservation Ecology Center, Smithsonian Conservation Biology Institute; and Michael Kaspari, Department of Biology, Geographical Ecology Group, University of Oklahoma.

Read more in Science and Nature