Plants Pollinated by Non-Native Bees Produce Worse Offspring

A new study from researchers at the University of California San Diego (UC San Diego) suggests that the plants produced following pollination by non-native honey bees tend to be of lower quality than those resulting from native pollinators. Published in Proceedings of the Royal Society B, the study is thought to be the first to directly compare offspring quality in plants pollinated by honey bees versus other pollinators. 

Honey bee heaven

The humble honey bee is the most frequent floral visitor worldwide, accounting for around 13% of all visits to native vegetation. Native to Europe, western Asia and Africa, the honey bee was first brought to the Americas in the 17th century. 

San Diego is one of the foremost honey bee hotspots in the United States, with a higher frequency of floral visits by honey bees than nearly anywhere else in the world. This “honey bee heaven” features more than 650 native species of bee and other pollinating insects, interacting with at least 2,400 different types of plants.  

Previous research has indicated that predominantly feral honey bees account for more than 90% of the pollinators observed visiting flower blooms around San Diego. To learn more about the impact these dominant honey bees have on the region’s floral diversity, Professor Joshua Kohn and recent graduate student Dillon Travis from the UC San Diego School of Biological Sciences conducted a series of experiments to evaluate the behavior of honey bees and the fitness of plants produced as a result of their pollinating efforts.

Feral bees result in more self-pollination

The researchers observed that honey bees visit approximately twice as many flowers on a given plant before moving onto the next one, compared to native insect visitors. This methodical foraging behavior is concerning as it can be risky for plant health – it means that most of the pollen being delivered to flowers will come from the same individual plant. Such self-pollination may create lower-quality offspring. 

To formally assess the effects of honey bees’ pollination dominance and their tendency to visit multiple flowers on the same plant, the researchers studied the fitness of plant offspring produced by three common plant species from native habitats in San Diego County. The researchers assessed a variety of conditions, including natural pollination, no pollination, honey bee pollination, native bee pollination and self- and cross-hand pollination. 

Four to six weeks post-pollination, the researchers collected the seeds and compared various factors relating to their maturation, germination, survival, growth and reproduction. 

They found that offspring resulting from pollination by native insects, mostly native bee species, were between two to five times more fit – meaning more likely to mature into seed, germinate, grow and reproduce – than those pollinated by honey bees. Offspring resulting from hand pollination using pollen from the same individual plant were also 2 to 10 times less fit than the offspring produced from pollen from a different plant of the same species. This suggests that the loss of fitness seen with honey bee pollination is a result of this self-pollination issue.  

“While honey bees are perceived as beautiful mutualists that are helping plants with reproduction, it turns out they may not be as good for plants as many native pollinators,” said Kohn, a professor in the Department of Ecology, Behavior and Evolution. “We have found that they deliver lower quality pollen than do native pollinators.”

Non-native bees could be putting plants at risk

In another related study (currently in press), Travis and Kohn report that honey bees were found to visit more flowers per plant than average among other pollinators across 44 different native plant species, which included both crop and non-crop plants. Based on this, the researchers say that the foraging behavior of honey bees may be regularly delivering more self-pollen, which results in lower-quality offspring across these plant species.

“If honey bees generally lower seed fitness of native plants, it could make the native plant community more susceptible to invasion from introduced plant species that do not require insect pollination […]” the researchers note in their paper. Introduced plant species include grasses and other invasives that often help spread wildfires in these ecosystems.

“People see honey bees as providing a valuable service, which is pollination, but there’s a decent amount of evidence to show that they’re competing with native insects for resources like pollen and nectar,” said Travis, who indicated that honey bees are also known to have viruses that can be transferred to native bees. “Many conservation efforts are focused on saving the honey bee, but they are not in any danger of going extinct. In fact, their numbers have been increasing. The organisms that do need our help are the native plants and bees.”

Reference: Travis DJ, Kohn JR. Honeybees (apis mellifera) decrease the fitness of plants they pollinate. Proc R Soc B. 2023;290(2001):20230967. doi: 10.1098/rspb.2023.0967

This article is a rework of a press release issued by the University of California San Diego. Material has been edited for length and content.