Voracious Honey Bees Threaten the Food Supply of Native Pollinators

Because honey bees are larger than most native bee species in Southern California, the new study calculated that honey bees now comprise 98% of all bee biomass in this ecosystem. If the pollen and nectar used to create honey bee biomass were instead converted to native bees, populations of native bees would be expected to be roughly 50 times larger than they are currently. 

“Although honey bees are rightly considered an indispensable asset to humans, they can also pose a serious ecological threat to natural ecosystems where they are not native,” said Hung, who earned his PhD from UC San Diego and is now an assistant professor at the University of Oklahoma. “The plight of the honey bee is an issue of animal husbandry and livestock management, whereas when it comes to conservation issues here in North America, honey bees are likelier to be part of the problem, not a solution or a target for conservation.”

In another cause for concern, a study published in 2023 by Travis and Kohn showed that plants pollinated by honey bees produce lower-quality offspring compared with offspring from native pollinators.

While bees in general are being threatened by habitat loss, climate change and chemical pollution, the researchers say that such a level of honey bee pollen exploitation is not well documented, and could well pose an additional and important threat to native bee populations in places where honey bees have become abundant. Even as the number of managed honey bee colonies is increasing worldwide due to the commercial beekeeping industry, many species of native pollinators are declining. “Public concern for honey bees often fails to consider their potential negative effects on native pollinators,” the authors note in their report.

“Honey bees are incredibly effective at extracting resources like pollen and nectar,” said Travis, who earned his PhD at UC San Diego in 2023. “Unlike the vast majority of native bee species in the region, honey bees can communicate to their nestmates the locations of rewarding plants and quickly remove most of the pollen, often early in the morning before native bees begin searching for food.”

The new study used pollen-removal experiments to estimate the amount of pollen extracted by honey bees using three common native plants (black sage, white sage and distant phacelia — also known as distant scorpion weed) as targeted pollen sources. The researchers found that just two visits by honey bees removed more than 60 percent of the available pollen from flowers of all three species. Such prodigious rates of pollen exploitation leave scant pollen for the more than 700 species of native bees in the region.

“The most surprising finding was the extraordinarily small number of individual native bees observed that were as large or larger than honey bees,” said Professor Emeritus Kohn of the Department of Ecology, Behavior and Evolution.  “Particularly rare were bumble bees, which made up only 0.1% of all bees we observed.”

With the new study shedding light on the steep honey bee/native bee imbalance in San Diego and Southern California, the researchers say resource consumption by honey bees should receive greater attention as a potential factor in pollinator declines. One step to address the situation could be increased guidance on whether and where large-scale contract beekeepers are allowed to keep their hives on public lands after crops have bloomed, to limit opportunities for honey bees to outcompete native species for scarce resources provided by native vegetation.

“In areas with threatened bee species, natural preserve managers may also want to consider systematic removals or relocations of non-native honey bee colonies to provide wild bees a fighting chance,” said Hung.

The research was supported by the Sea and Sage Audubon Society, the Jeanne Marie Messier Memorial Endowed Fund, a University of California Multi-campus Research Programs and Initiatives grant, a National Science Foundation Doctoral Dissertation Improvement Grant (DEB-1501566), a Mildred E. Mathias Graduate Student Research Grant, an Institute for the Study of Ecological and Evolutionary Climate Impacts Graduate Fellowship from the UC Natural Reserve System, a Frontiers of Innovation Scholar Fellowship, a McElroy Fellowship from UC San Diego and the California Native Plant Society.