The honey bees love it.

So do the long-horned bees, bumble bees, carpenter bees, European paper wasps, syrphid flies, butterflies, blister beetles, spotted cucumber beetles, crab spiders, praying mantids, and assorted other insects.

The Mexican sunflower (Tithonia rotundifolia) blooming in gardens around California and beyond is a delight to see. 

A native of Mexico and Central America and an annual, it's a member of the sunflower family Asteraceae. In our yard in Vacaville, Calif, it blooms from May or early June through October and November--just in time for the migrating monarchs that pass through on their way to their overwintering sites along coastal California.

But for now, it belongs to the honey bees and the long-horned bees, such as Melissodes agilis. 

We encountered this lone honey bee last week--a single bee in need of nectar but not in need of a dive-bombing by the male territorial Melissodes agilis.

The last image, of her in an upside-down stance and peering through the petals, indicates this bee is not about to let her guard down. 

Not now. 

Want to learn about honey bees? Be sure to read Norman Gary's book, The Honey Bee Hobbyist: The Care and Keeping of Bees. Gary, a UC Davis emeritus professor of entomology, has kept bees for more than seven decades and has held or holds the titles of teacher, scientist, researcher, author, bee wrangler and musician. Check out his website.

Also read the UC book, California Bees and Blooms: A Guide for Gardeners and Naturalists, by Gordon Frankie of UC Berkeley, (the late) Robbin Thorp of UC Davis, and Rollin Coville and Barbara Ertter, both affiliated with UC Berkeley.

The things we overlook are the things we should look for.

Take mustard and honey bees.

You've seen mustard thriving in fields, but have you ever considered planting some seeds from a nursery in your garden so honey bees will have something to eat in early spring? (And then, of course, replacing the mustard with other plants so the mustard does not go to seed? The soil also benefits.)

We've been planting mustard now for several years, and it's a joy to watch the bees foraging, flying and gathering pollen and nectar. Sometimes they'll stop in mid-air to clean their tongues.

"The importance of pollen to the health and vigor of the honey bee colony cannot be overstated," writes Norman Gary, emeritus professor of entomology at the University of California, Davis, in his book, Honey Bee Hobbyist: The Care and Keeping of Bees. "Honey bees need a balanced diet. Honey satisfies the  bees' carbohydrates requirements, while all of the other nutrients--minerals, proteins, vitamins and fatty substances--are derived from pollen. Nurse bees consume large amounts of pollen, converting it into nutritious secretions that are fed to developing larvae. During an entire year, a typical bee colony gathers and consumes around 72 pounds of pollen."

"Pollen in the plant world is the equivalent of sperm in the animal world," Gary continues. "Fertilization and growth of seeds depend upon the transfer of pollen from the male flower parts (anthers) to the receptive female parts (stigma)."

Yes, airborne pollen causes most human allergies, Gary acknowledges.

But for us, watching the bees go airborne with a load of pollen is a treat, a treasure and a tribute to their industrious bee-ings.

You're watching honey bees foraging in a field. 

They buzz toward a blossom, sip nectar, and then head for another blossom. Typical, right?

But there's much more going on than you think.

It's not just the nectar that she's scented.

UC Davis community ecologist Rachel Vannette has just published a paper in New Phytologist journal that shows nectar-living microbes release scents or volatile compounds, too, and can influence a pollinator's foraging preference.

The groundbreaking research shows that nectar-inhabiting species of bacteria and fungi “can influence pollinator preference through differential volatile production,” said Vannette, an assistant professor in the UC Davis Department of Entomology and Nematology.

“This extends our understanding of how microbial species can differentially influence plant phenotype and species interactions through a previously overlooked mechanism,” Vannette said. “It's a novel mechanism by which the presence and species composition of the microbiome can influence pollination.”

“Broadly, our results imply that the microbiome can contribute to plant volatile phenotype,” she said. “This has implications for many plant-insect interactions.”

Their paper, titled “Nectar-inhabiting Microorganisms Influence Nectar Volatile Composition and Attractiveness to a Generalist Pollinator,” may explain in part the previous documented extreme variation floral volatiles that Robert Junker of University of Salzburg, Austria, and his team found; New Phytologist published their work in March 2017.

In their study, the Vannette team researchers first examined field flowers for the presence of nectar-inhabiting microbes, and in collaboration with co-authors Caitlin Rering and John Beck of the U.S. Department of Agriculture's Agricultural Research Service (USDA-ARS), Gainesville, Fla, characterized the headspace of four common fungi and bacteria in a nectar analog. Next, they used an intricate setup to quantify the antennal and behavior responses of honey bees to the chemical compounds. Finally, when they examined the scent of flowers in the field, they found that flowers which contained high densities of microorganisms also contained volatile compounds likely produced by those microbes, suggesting that microbial scent production can be detected and used by pollinators.

Although microbes commonly inhabit floral nectar, microbial species differ in volatile profiles, they found. “Honey bees detected most of the microbial volatiles or scents that we tested,” Vannette said, “and they distinguished the solutions of yeasts or bacteria based on volatiles only.” This suggests that pollinators could choose among flowers based on the microbes that inhabit those flowers.

The yeast Metschnikowia reukaufii produced the most distinctive compounds (some shared with the fruity flavors in wine) and was the most attractive of all microbes compared. This yeast is commonly found in flower nectar and is thought to hitch a ride on pollinators to travel from one flower to the next. Its scent production may help it attract pollinators, which then help the yeast disperse among flowers.

The Harry H. Laidlaw Jr. Honey Bee Research Facility, UC Davis, provided the honey bees. More than 20 species of flowers--mostly natives--were used in the survey, including canyon delphinium or canyon larkspur (Delphinium nudicaule), sticky monkey flower (Mimulus aurantiacus), salvia (Lepechinia calycina) and purple Chinese houses (Collinsia heterophylla). The samplings were done in the spring and early summer, when the natives are at their peak.

Co-authors of the paper are Caitlin Rering, postdoctoral fellow at USDA-ARS, Gainesville, Fla.; John Beck researcher at USDA-ARS; Griffin Hall, junior specialist in the Vannette lab; and Mitch McCartney in UC Davis Department of Mechanical and Aerospace Engineering.

The USDA and USDA-ARS funded the research.

See abstract
See full paper

About Rachel Vannette: She joined the UC Davis Department of Entomology and Nematology in September of 2015 from Stanford University where she was a postdoctoral fellow.

A native of Hudsonville, Mich., Vannette received her bachelor of science degree in biology with honors at Calvin College, Grand Rapids, Mich., and her doctorate in ecology and evolutionary biology from the University of Michigan, in 2011. Her dissertation was entitled “Whose Phenotype Is It Anyway? The Complex Role of Species Interactions and Resource Availability in Determining the Expression of Plant Defense Phenotype and Community Consequences.”

In her PhD research, she examined how variation in nutrient availability and plant associations with mycorrhizal fungi belowground influenced defense chemistry in milkweed plants and the performance of a specialist herbivore (Danaus plexippus). She found that resource-based tradeoffs can in part explain plant allocation to antiherbivore defense and mycorrhizal fungi. This work also describes that plant genotypes vary in their investment in defense and associations with below ground fungi.

As a Stanford University postdoctoral fellow, funded by a life sciences research fellowship, Vannette examined the community ecology of plant-associated microorganisms. Using diverse systems, she studied the assembly of microbial communities, microbial response to anthropogenic changes like habitat fragmentation, and microbial effects on plant-pollinator interactions.

The National Wildlife Research Foundation featured Vannette's research on monarchs and milkweed in its March 11, 2013 piece on “Catering to Butterfly Royalty." The article, by author Doreen Cubie, focused on Vannette's research as a graduate student at the University of Michigan. Vannette and advisor Mark Hunter studied five common species of milkweeds, the host plant for monarchs. They found that climate change may disrupt the chemistry of milkweeds, and encouraged gardeners to help the monarchs by planting more of these critical host plants.

It's surprising what the microbes in nectar can reveal.

Take the nectar of the sticky monkeyflower, Mimulus auranticus.

UC Davis community ecologist Rachel Vannette and colleague Tadashi Fukami of Stanford University decided to examine microbial communities inhabiting the nectar of the sticky monkeyflower at the Jasper Ridge Biological Preserve in California's Santa Cruz Mountains.

Their paper, "Dispersal Enchances Beta Diversity in Nectar Microbes," published in Ecology Letters, revealed that contrary to popular assumption, “increased dispersal among habitats can actually increase biodiversity rather than decrease it."

The flower, in the family Phrymacease, is a native shrub common in chaparral and coastal scrub habitats of California and Oregon. It is primarily pollinated by Anna's hummingbird. Other common pollinators include bumble bees, carpenter bees, and thrips.

Dispersal is considered a key driver of beta diversity, which is “the variation in species composition among local communities,” Vannette said.

They are the first to publish work showing that increased dispersal can increase biodiversity. 

In their experiment, they reduced natural rates of dispersal by eliminating multiple modes of microbial dispersal. “Specifically we focused in nectar-inhabiting bacteria and yeasts that are dispersed among flowers by wind, insects and birds,” they said. “We imposed dispersal limitation on individual flowers and quantified microbial abundance, species composition and microbial effects on nectar chemistry.”

The scientists “manipulated dispersal of nectar-inhabiting bacteria and yeasts via flower-visiting animals to examine how dispersal influenced microbial beta diversity among flowers. Our analysis suggested that this unexpected pattern might have resulted from stronger priority effects under increased dispersal.”

This work has direct implications for conservation of many organisms in addition to bacteria and yeast, suggesting that preserving routes of dispersal among habitat patches may be important in the maintenance of biodiversity. In contrast to previous work showing that dispersal can homogenize communities or make them more similar, the published work demonstrates that dispersal can in some cases generate communities that are more different from each other. The authors hypothesize that this could be driven by priority effects, where early arriving species change the species that can establish within that habitat.

Why focus on nectar-inhabiting microbes? Previous work by Vannette and others shows that microbial activity in nectar can alter nectar chemistry and influence plant-pollinator interactions by altering nectar chemistry. In the Ecology Letters study, microbes were also found to change nectar chemistry, explaining ~50% of the variation in sugar composition in the field. This suggests that nectar-inhabiting bacteria and yeast can influence the nectar rewards available to pollinators in a natural setting.

More broadly, “Studying the role of microbes in the environment addresses one of the biggest mysteries in science,” Vannette says. In her current work, she and her lab are investigating how microbial communities form, change, and function in their interactions with insects and plants. They are also researching how microorganisms affect plant defense against herbivores and plant attraction to pollinators.

Vannette, a former postdoctoral fellow at Stanford, joined the UC Davis Department of Entomology and Nematology faculty as an assistant professor in 2015.

Vannette's research was funded by the Gordon and Betty Moore Foundation through the Life Sciences Research Fellowship. Stanford also funded the research through grants from the National Science Foundation, the Terman Fellowship, and the Department of Biology at Stanford University.