The western honey bee (Apis mellifera), the world’s most important pollinator for agriculture, is facing a crisis. Parasitic mites, colony collapse and climate change threaten hives. California, as the seasonal home of nearly half of the continental United States’ managed honey bee colonies, is a prime location for monitoring bee populations. And honey bee health, key to the nation’s largest fresh produce economy, is vital to the Golden State. 

In a study appearing in Genome Biology and Evolution, Assistant Professor Santiago Ramirez, Department of Evolution and Ecology, and postdoctoral researcher Julie Cridland provide a genetic snapshot of the state’s honey bee populations, defining how the species has changed over the past 105 years. Their findings could help researchers breed hardier honey bees capable of thriving under many environmental stressors.

“California depends on honey bees for pollination services,” said Ramirez. “Understanding the amount and distribution of genetic variation of honey bee populations in California is critical to understanding their current state and ensuring their future as a managed species for agricultural purposes.”  

Investigating the bee family tree

Cridland and Ramirez found evidence of substantial genetic changes in both Northern and Southern California honey bee populations. With their colleagues, they examined the genomes of historical museum specimens and of bees from modern California populations during the course of their research. 

Both western European and eastern European honey bees were brought to California in the 1850s. The researchers found that since the 1960s, the lineages of Northern California honey bees shifted from a western European ancestry to that of an eastern European ancestry, reflecting the preference of apiculturists. 

Southern California honey bees show traits of a different lineage. Africanized honey bees, originally imported into Brazil to bolster honey production, were first documented in California in 1994. Since then, traits associated with the hybrid species have spread throughout Southern California’s honey bees, the researchers found. 

A new take on Africanized bees

Africanized honey bees are known to be more aggressive and produce less honey than their European counterparts, but despite their negative portrayal in popular media, they’re important pollinators. Additionally, they’re able to survive on minimal nutrients and are highly adaptable to their environment.

“The Africanized bees in Southern California are more genetically diverse and are possibly hardier than Northern California bees, which are predominantly derived from managed populations,” said Cridland. “There’s also some evidence suggesting that Africanized bees are more resistant to Varroa mites.”

Accidentally introduced in the 1980s, Varroa mites spread prolifically and devastated feral honey populations throughout the state. It’s thought that Africanized bees are resistant to the mites due to higher grooming rates and shorter periods between egg laying and larval growth.

Cridland speculates that these Africanized traits could help bolster the hive strength of California’s honey bees.  

Evidence suggests that genetic diversity improves the overall health of bee colonies. Genetically diverse hives have exhibited higher disease resistance and respond more effectively to different environmental stimuli.  

Breeding a hardier bee

Uncovering the genetic shifts occurring in California honey bee populations not only gives scientists a picture of the organism’s rapid and recent evolution but provides insights into how researchers might help ensure the species’ survival.

“This type of analysis may help improve breeding practices to overcome current issues with high levels of mortality in honey bees as caused by Varroa mites, pesticides and poor nutrition,” said Ramirez.

With their genetic diversity, Southern California honey bees could help improve the health and resiliency of the species overall and ensure a productive future for California’s agricultural industry, which feeds much of the country.

Coauthors on the paper were Cheryl Dean, manager of the Ramirez Lab, and researchers Amber Sciligo and Neil D. Tsutsui of UC Berkeley. The research was supported by the Berkeley Initiative for Global Change Biology, the Office of the Vice Chancellor for Research at UC Berkeley, the Gordon and Betty Moore Foundation, the Vincent Coates Genome Sequencing Facility, the National Institutes of Health and the USDA National Institute of Food and Agriculture.