UC Davis Research Shows That Yeast Can Benefit Bumble Bee Survival and Reproduction

Bumble bees prefer foraging on flowers that contain yeast, but does yeast benefit bumble bee survival and reproduction?

Yes, according to UC Davis community ecologist and doctoral candidate Danielle Rutkowski and her colleagues in their newly published research in the Royal Entomological Society's Journal of Ecological Entomology.

The research, “Bee-Associated Fungi Mediate Effects of Fungicides on Bumble Bees,” provides direct evidence that fungi can benefit both survival and reproduction in two species of bumble bees, Bombus vosnesenskii,  and B. impatiens. The research also suggests that yeast, commonly found in the gut of bumble bees, may be more important than originally thought.

“Bumble bees are important pollinators that face threats from multiple sources, including agrochemical application,” said Rutkowsi, the lead researcher-author. “Declining bumble bee populations have been linked to fungicide application, which could directly affect the fungi often found in the stored food and gastrointestinal (GI) tract of healthy bumble bees.”

 “I tested if fungicides commonly applied in orchard systems affected yeasts and the health of their bee hosts, and if feeding those bees their fungi after fungicide exposure could rescue them,” said Rutkowski, who studies with major professors Rachel Vannette and Richard Karban, community ecologists in the Department of Entomology and Nematology.

“Bombus vosnesenskii (commonly known as the yellow-faced bumble bee), is native to California and we reared colonies of it from wild-caught queens,” Rutkowski said. “In this species, we observed strong negative effects of fungicide and the ability of bee-associated fungi to rescue bees from these negative effects. The other species, Bombus impatiens, is native to the eastern United States, but is commonly produced and sold commercially for pollination. In this species, we did not find any negative impacts of fungicide, but the addition of yeasts was very beneficial for bee survival and offspring production.” 

Vannette, a co-author of the paper, pointed out: “We knew that bumble bees in the field prefer foraging on flowers that contain yeasts, and that bumble bees have yeasts in their gut; but didn't know how those yeasts affected bees, or how fungicides affected yeast-bee interactions.”

“Although most previous work on bee microbiomes has focused on bacteria and their role in bee health, Danielle's work suggests that yeasts --which are commonly found in association with bumble bees--may be more important than previously thought,” Vannette said. “This has been hinted at in the literature but rarely tested directly.” 

Rutkowski examined the interactive effects of the fungicide propiconazole and fungal supplementation on the survival, reproduction and microbiome composition of microcolonies (queenless colonies) using the two species.

Both B. vosnesenskii and B. impatiens benefitted from fungal addition but in different ways. fungicide exposure decreased survival in B. vosnesenskii, while fungal supplementation mitigated fungicide effects. For B. impatiens, fungicide application had no effect, but fungal supplementation improved survival and offspring production.

The research also shows that for at least some bumble bee species, fungi can mediate the effects of fungicides on bee health. “In other words, the fungicide tested here (propiconazole) may harm the bee-associated fungi rather than the bees themselves, and restoring the yeasts (a type of fungus) can mitigate effects on bee survival,” Rutkowski said. 

 “Because the effect of fungicides on yeasts and bees takes a few weeks to observe, it is not detected in short term LD50 trials,” Vannette said, “and therefore could be an unrecognized threat to bumble bees and their symbionts.”

Other co-authors of the paper are entomologist Eliza Litsey and environmental scientist Isabelle Maalouf. 

More research is planned to determine the mechanism by which yeasts can affect bee health, and which fungicides affect bee-associated yeasts. 

 “I'm currently working on a project to determine the mechanisms behind the positive effects of yeast addition that we observed,” Rutkowski said. “In some bees and other insects, fungi can produce nutritionally important compounds for their host, and I'm currently trying to determine if this is the case for bumble bees as well.” 

“Additionally, I'm planning on following up some of the interesting results on differences between bumble bee species by determining how associated microbial communities differ between wild and commercially-reared bumble bees,” Rutkowski said. “In this current paper, we found that the fungal communities associated with the commercially-sourced bees were less diverse and less abundant, and I'm hoping to determine if that's a common pattern.” 

Rutkowski, who joined the UC Davis doctoral program in 2018, won the President's Prize (first-place) in her category for her graduate student research presentations at the 2017 and 2021 Entomological Society of America meetings. A 2018 graduate of Cornell University, summa cum laude, she holds a bachelor of science degree in entomology and biological sciences, with a concentration in ecology and evolution.

Rutkowski's research drew support from her three-year USDA National Institute of Food and Agriculture grant.