SoCal honeybees can fend off deadly mites

Southern California is home to a flying black and yellow treasure. While commercial honeybee hives nationwide are collapsing under attack from deadly parasites, a unique hybrid bee found only in this part of the state has demonstrated the ability to survive.

Southern California hybrid honeybee hive hanging from a tree in the wild. (Boris Baer/UCR)

U.S. beekeepers reported losing up to 62% of their managed honeybee colonies in 2025, which threatens our food supply. The losses are driven by a combination of pesticides, climate pressure, habitat loss, and parasites, with the Varroa mite among the most destructive of these factors.

Varroa mites feed on honeybees' fat body tissue, which weakens their immune systems, reduces their body weight, and shortens their lives. The fat body is an organ in a honeybee that stores energy, helps fight infections, and keeps the bee healthy, similar in some ways to the human liver.

The mites also act as vectors for deadly viruses like Deformed Wing Virus and Acute Bee Paralysis Virus, which they transmit directly into a bee's bloodstream. Beekeepers rely on chemical treatments for suppression that can lose effectiveness over time.

A new study from UC Riverside published in Scientific Reports is the first to show that a locally adapted population of honeybees can naturally and consistently suppress the mites.

"We kept hearing anecdotally that these Californian honeybees were surviving with way fewer treatments. I wanted to test them rigorously and understand the driving force behind what the beekeepers were seeing," said Genesis Chong-Echavez, a UCR graduate student and lead author of the study.

Alongside entomologists from UCR's Center for Integrative Bee Research (CIBER), Chong-Echavez monitored 236 honeybee colonies between 2019 and 2022.

A Varroa mite on a developing honeybee larva inside a brood cell. (Genesis Chong-Echavez/UCR)

The Californian bees were not entirely immune to the mites. However, colonies headed by locally raised Californian hybrid honeybee queens had about 68% fewer mites on average than colonies headed by commercial honeybee queens. They were also more than five times less likely to cross the threshold at which chemical treatments become necessary.

The bees in the study are not a commercial breed. They come from a genetically mixed population of honeybees established in Southern California, often from feral colonies living in trees. Recent research shows they are a hybrid population with ancestry from at least four honeybee lineages, including African, Eastern European, Middle Eastern, and Western European bees.

To more fully understand the bees' resistance to the mites, the researchers also ran laboratory experiments with developing honeybee larvae. Varroa mites must enter brood cells to reproduce, so the team tested whether mites were equally drawn to larvae from commercial and Californian hybrid honeybee colonies.

They were not.

Mites were less attracted to the Californian hybrid honeybee larvae, especially at seven days old, the stage when mites are normally most likely to invade. The finding suggests the bees' secret to fending off mites lies in early development, before any adult worker behaviors might come into play.

"What surprised me most was the differences showed up even at the larval stage," Chong-Echavez said. "This suggests the resistance mechanism may go deeper than some kind of behavior and may be genetically built into the bees themselves."

Researchers inspecting honeybee colonies as part of long-term monitoring of Varroa mite infestations. (Genesis Chong-Echavez/UCR)

The findings could have implications beyond Southern California. Honeybees pollinate crops worth billions of dollars and are under growing pressure from multiple environmental stressors. The research suggests that part of the answer to improving honeybee health may lie in the biology of these bees.

Boris Baer, UCR entomology professor and co-author of the study, said the study also highlights the value of listening to working beekeepers.

"This question did not start in the lab. It started in conversations with beekeepers," Baer said. "They were not just observers; they helped shape the questions behind this research."

The researchers caution that the Californian hybrid honeybees are not entirely mite-free, and they do not suggest abandoning current management practices. Instead, they hope to learn which traits help these honeybees keep mite levels lower, and whether those traits could support future breeding programs or reduce dependence on chemicals.

Next, the team plans to investigate the genetic, behavioral, and chemical signals that may make the larvae less attractive to mites.

"At a time when pollinators are facing global decline, this work offers a hopeful message: solutions may already be emerging in the field, and we just need to understand them," Chong-Echavez said.

(Cover image of honeybees: Irina Shilnikova/iStock/Getty)

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