Lisa Tell

Rachel Vannette

(Proceedings of the Royal Society B)

DAVIS--If you're like many Americans, you've set up a hummingbird feeder in your yard to nurture and watch these high-energy pollinators. But are you concerned that the sugar water you provide might be impacting your tiny feathered friends?

Newly published research indicates that sugar water in hummingbird feeders can contain high densities of microbial cells but “very few of the bacteria or fungi identified have been reported to be associated with avian disease,” says community ecologist and co-author Rachel Vannette of the University of California, Davis.

The research, published in the Proceedings of the Royal Society B, is one of the first to explore the microbial communities that dwell in sugar water from feeders and compare them to those found in flower nectar and samples from live hummingbirds.

“The potential for sugar water from hummingbird feeders to act as a vector for avian pathogens--or even zoonotic pathogens--is unknown,” said Vannette, an assistant professor in the UC Davis Department of Entomology and Nematology. “Our study is one of the first to address this public concern. Although we found high densities of both bacteria and fungi in sugar water samples from feeders, very few of the species of bacteria or fungi found have been reported to cause disease in hummingbirds.”

“So although birds definitely vector bacteria and fungi to feeders, based on the results from this study, the majority of microbes growing in feeders do not likely pose significant health hazards to birds or humans,” Vannette said. “However, a tiny fraction of those microbes has been associated with disease, so we encourage everyone who provides feeders for hummingbirds to clean their feeders on a regular basis and to avoid cleaning feeders in areas where human food is prepared.”

The paper, “Microbial Communities in Hummingbird Feeders Are Distinct from Floral Nectar and Influenced by Bird Visitation,” is the work of first author Casie Lee, a UC Davis School of Veterinary Medicine student; Professor Lee Tell of the UC Davis School of Veterinary Medicine's Department of Medicine and Epidemiology; Tiffany Hilfer, an undergraduate student and Global Disease Biology major; and Vannette.

Lee, mentored by Vannette and Tell, led the field experiment and performed bird observations and laboratory work during a summer project funded by the Students Training in Advanced Research (STAR) and Merial Veterinary Scholars Programs.

The researchers also compared the microbes in the feeders to those in floral nectar and found they differed in microbial composition.

“Birds, feeder sugar water, and flowers hosted distinct bacterial and fungal communities,” they wrote in their abstract. “Floral nectar and feeder sugar water hosted remarkably different bacterial communities; Proteobacteria comprised over 80% of nectar bacteria, but feeder sugar water contained relatively high abundance of Firmicutes and Actinobacteria, as well as Proteobacteria. Hummingbird feces hosted both bacterial taxa commonly found in other bird taxa and novel genera including Zymobacter (Proteobacteria) and Ascomycete fungi.”

The UC Davis scientists conducted their research at a private residence in Winters, attracting two hummingbird species, Calypte anna (Anna's Hummingbird) and Archilochus alexandri (Black-chinned Hummingbird) to drop net feeder traps. They mixed bottled water with conventional white granulated sugar (one part sugar and four parts water).

The researchers assigned feeders to one of three treatments including (1) access by both hummingbirds and insects (open feeders), (2) restricted access by birds but access by insects allowed (caged feeders, 1.5 cm square mesh), or (3) restricted access by both birds and insects (feeders bagged using gallon paint strainer bags), with two replicates of each treatment set up at each site, and the setup was replicated four times throughout the summer.

They examined and characterized microbial communities on bills and fecal material of the two species, as well their food resources, the feeder sugar water, and floral nectar. They tested sugar water from feeders that were visited and not visited. Results indicated that “both accumulated abundant microbial populations that changed solution pH and bird visitation rates.” Although both feeder sugar water and flowers contained aerobic, sugar-loving bacteria, they found that floral nectar contained mostly bacteria found only in flowers (Acinetobacter and Rosenbergiella) while feeder sugar water contained generalist bacteria that grow in many types of aqueous environments.

In pointing out that the microbial populations in the feeders differed from those in natural floral nectar, the UC Davis scientists noted that “human provisioning (of sugar water in feeders) influences microbial intake by free-ranging hummingbirds; however, it is unknown how these changes impact hummingbird gastrointestinal flora or health.”

As part of the study, they also performed a small experiment to assess how water type influences microbial growth. When feeders were exposed to birds, they found that deionized water supports the most fungal growth while tap water or bottled water supports the most bacterial growth.

“The microbes that hummingbirds are eating depends a lot on bird diet-- if they have access to feeders or are just consuming floral nectar,” Vannette said. “We don't know what the consequences are for bird health or gastrointestinal flora but we think that there should be more studies examining this, as many, many people use feeders, and the birds are opportunistic and drink from feeders!”

Hummingbirds (family Trochilidae) are one of the world's few avian pollinators. “Nearly 15% of hummingbird species are threatened or endangered,” the scientists related, so “understanding drivers of health and population dynamics may help conservation efforts. Avian microbial associates are just beginning to be studied in depth.”

Professor Tell said that “although our study does not directly inform hummingbird health outcomes, shifts in microbial composition in bird diets may influence bird microbiomes as a consequence. In the future, it will be important to understand how consumed microbial populations could potentially influence the health of free ranging hummingbirds, particularly with regards to anthropogenic effects on wildlife.”

Tell emphasized that the ideal human-provisioned food source for hummingbirds is floral nectar. “However, if feeders are offered, best practices entail routine and thorough cleaning that does not result in harmful residues,” she said.

Lee, who previously worked in wildlife rehabilitation and studied ecology before enrolling in the UC Davis School of Veterinary Medicine, commented: “I was very excited to work on a project that examined health of backyard wildlife in the context of modern society and grateful to have done it in the company of such hummingbird enthusiasts.”

Vannette was recently named one of 11 campus recipients of Hellman Fellowship grants, awarded to assistant professors who exhibit potential for great distinction in their research. Her project, “Characterizing the Structure and Function of Pollinator Microbiomes,” involves investigating the communities of bacteria and fungi in flowers and pollinators, including bees and hummingbirds. “Our work to date suggests that microbes in flowers are common and influence pollinator behavior,” she said.

DAVIS--Community ecologist Rachel Vannette, assistant professor, UC Davis Department of Entomology and Nematology, is one of the 11 campus recipients of this year's Hellman Fellowship grants.

The Hellman Family Foundation contributes funds to support and encourage the research of promising assistant professors who exhibit potential for great distinction in their research. The fellowship is designed to support research and creative activities that will promote career advancement.

Vannette's project, “Characterizing the Structure and Function of Pollinator Microbiomes,” will investigate the communities of bacteria and fungi in flowers and pollinators including bees and hummingbirds. “Our work to date suggests that microbes in flowers are common and influence pollinator behavior,” says Vannette. The current funding will allow her to link microbial communities in flowers with their influence on pollinators by examining microbial modification of nectar and pollen chemistry, and examine how microbial effects vary among plant and pollinator species, and with environmental variation.

The 11 assistant professors will receive a total of $244,000 in grants for research in a wide range of disciplines. Since 2008, UC Davis has received nearly $3 million in Hellman grants, awarded to 136 early-career faculty members. The Hellman Fund provides grant monies to early career faculty on all 10 UC campuses, as well as to four private institutions.

Vannette is one of two recipients from the College of Agricultural and Environmental Sciences. The other is Frances Moore, an assistant professor, Environmental Science and Policy. Her project title: "Quantifying the Costs of Ecosystem Damages from Climate Change for Improved Climate Policy Analysis.”

The community ecologist joined the UC Davis Department of Entomology and Nematology in 2015 after serving as a postdoctoral fellow at Stanford University's biology department, where she was a Gordon and Betty Moore Foundation Postdoctoral Fellow from 2011 to 2015 and examined the role of nectar chemistry in community assembly of yeasts and plant-pollinator interactions.

Vannette received her bachelor of science degree, summa cum laude, in 2006 from Calvin College, Grand Rapids, Mich., and her doctorate from the University of Michigan's Department of Ecology and Evolutionary Biology, Ann Arbor, in 2011. Her thesis: “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.”

(Editor's Note: See UC Davis Dateline story on 11 recipients)

Elina Niño, co-coordinator

Rachel Vannette, co-coordinator

The UC Davis Department of Entomology and Nematology has announced the list of seminars it will host for the winter quarter.

The seminars begin Jan. 10 and will continue through March 14. All will take place on Wednesdays from 4:10 to 5 p.m. in 122 Briggs Hall, Kleiber Hall Drive.

Coordinators are assistant professor Rachel Vannette; Extension apiculturist Elina Niño;  and Ph.D student Brendon Boudinot of the Phil Ward lab.

The schedule (subject to change):

Jan. 10: Amy Morrison, UC Davis epidemiologist, project scientist and scientific director of the Naval Medical Research Unit No. 6 (NAMRU-6) Iquitos Laboratory. Topic:  "Targeting Aedes Aegypti Adults for Dengue Control: Infection Experiments and Vector Control in Iquitos." 

Jan. 17: Fiona Goggin, professor of entomology, University of Arkansas and a UC Davis alumnus. Topic: “Molecular and Phenomic Approaches to Study Plant Defenses against Insects and Nematodes."

Jan. 24: David Gonthier, postdoctoral fellow, Clare Kremen lab, UC Berkeley. Topic: to be announced. His primary research objective is to understand the importance of biodiversity across natural and managed ecosystems.

Jan. 31: Amanda Hodson, UC Davis postdoctoral fellow and assistant professional researcher with Louise Jackson's Soil Ecology Lab, UC Davis. Topic: "Molecular Detection and Integrated Management of Plant Parasitic Nematodes." Her research interests include soil ecology, integrated pest management and ecological intensification of agricultural systems.

Feb. 7: Marm Kilpatrick, assistant professor, Department of Ecology and Evolutionary Biology, UC Santa Cruz. Topic: to be announced. He studies ecology of infectious diseases and population biology.  His research "unites theory and empirical work to address basic and applied questions on the ecology of infectious diseases as well as population biology, evolution, climate, behavior, genetics, and conservation."

Feb. 14: Maj Rundlöf of Lund University, Sweden, a visiting International Career Grant (INCA) fellow in the Neal Williams lab, will speak on “Pesticide Exposure and Flower Resources as Drivers of Bumble Bee Diversity in Agricultural Landscapes"

Feb. 21: Kerry Mauck, assistant professor of entomology, UC Riverside, will speak on “How Plant Viruses Use Chemistry to Manipulate Hosts and Vectors." She studies insect vector behavior, plant-pathogen interactions, chemical ecology, and integrated disease management.

Feb. 28:  Brendon Boudinot, Ph.D candidate in the Phil Ward lab, UC Davis Department of Entomology and Nematology, will speak on "Phylogenetic Morphology of the Big-Eyed Tree Ants and Kin (Formicidae: Pseudomyrmecinae)."

March 7: John Tooker, associate professor of entomology and Extension specialist, Department of Entomology, Pennsylvania State. Topic: to be announced. His areas of expertise include insect ecology, plant-insect interactions, conservation biological control, chemical ecology and gall insects.

March 14: Alvaro Acosta-Serrano, senior lecturer, Liverpool School of Tropical Medicine. Topic: to be announced. His research focuses on fundamental aspects of the biology of kinetoplastid parasites and their vectors, and on developing molecular tools to control and prevent parasite transmission in disease-endemic areas.

For more information, contact Vannette at rlvannette@ucdavis.edu; Niño at elnino@ucdavis.edu or Boudinot at boudinotb@gmail.com.

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DAVIS--Hear that honey bee buzzing toward a flower? It's not just the nectar that she's scented.

Nectar-living microbes release scents or volatile compounds, too, and can influence a pollinator's foraging preference, according to newly published research led by UC Davis community ecologist Rachel Vannette.

The groundbreaking research, published in the current edition of New Phytologist journal, 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.