Every bumble bee colony has a queen, but a new study led by researchers at Penn State suggests the process of determining which baby bee reigns supreme may be less monarchal than the royal title suggests.
The study, published in the journal Insect Biochemistry and Molecular Biology, explored why some bumble bee larvae become workers and others become queens, despite coming from the same eggs.
The team found that juvenile hormone - a hormone in insects responsible for insect development, molting and reproduction - was key to this process. When researchers administered the hormone to worker bees, they passed it to all the larvae through feeding. The more hormone the larvae received, the more likely they were to become queens.
It's the first study to show that bumble bee caste is determined by the workers, shifting the understanding of the colony from a top-down hierarchy to a decentralized system where the caregivers can alter the future of baby bees, the researchers said.
Etya Amsalem, associate professor of entomology in the College of Agricultural Sciences and corresponding author on the study, said that understanding larval fate is key to understanding social behavior in the insects, which rely on reproductive division of labor: some females reproduce while others help.
"Since all these females share the same DNA, it's a striking example of how the same genotype can produce very different forms," she said. "It's also a practical question since bumble bees are important for pollination, so knowing how to produce queens could improve commercial breeding and management."
Queen bees and worker bees don't just have different social roles, they are also physically very different, the researchers said. Bumblebee queens are large, long-lived and reproductive, while workers are smaller and do not reproduce.
Seyed Ali Modarres Hasani, a postdoctoral researcher at Penn State and lead author on the paper, said that previously, scientists knew hormones were involved in the process that determined whether larvae become workers or queens, but the exact mechanisms were unknown.
"A single female egg in bumblebees holds the blueprint for two completely different life paths: the giant, reproductive queen or the small, sterile worker," he said. "We wanted to understand what triggers the change in the female life trajectory, when does it happen and who controls the process."
For the study, the researchers combined hormonal manipulation with chemical tracing in a tightly controlled setup: three worker bees and a cluster of larvae. They applied juvenile hormone at different doses and times, either to workers or directly to larvae, and traced the hormone's movement using chemical labels and advanced chromatography techniques at the Huck Institutes for the Life Sciences' Metabolomics Core Facility. They then measured larval mass and recorded which individuals became queens or workers.
"Every colony will produce many new queens at the end of the season," Amsalem said. "These queens will leave the colony, mate and go into winter diapause, and then each queen will start a new colony in the next spring. In that sense, producing as many queens - and males -at the end of the season is the ultimate purpose of the colony."
The researchers found that when juvenile hormone was applied directly to the larvae, not only did they not turn into queens, but the worker bees actually eliminated most of these larvae.
However, when workers were treated with the hormone, they incorporated it into the food they make for the larvae. These larvae ingested the hormone, ending up heavier and much more likely to become queens as a result.
"We also determined that larvae are only sensitive to this hormone on days seven and eight of their development," Hasani said. "By tracing the juvenile hormone, we saw that the workers pass the hormone into the food they make from nectar and pollen."
Amsalem said their results suggest that queen production is linked to how the colony progresses through the warmer months until it collapses in the fall.
"Bumblebee workers do not reproduce when the colony is young, but they can activate their ovaries and produce males as the colony ages, which causes an increase in juvenile hormone levels," she said. "As a result, over time, they feed larvae more of the hormone. When enough workers do this simultaneously, usually towards the end of the season, larvae receive doses that are high enough during the critical window to develop into queens."
The researchers said the findings provide a framework for understanding hormonal regulation of caste in social insects and could eventually help improve bumblebee management. More broadly, the work could also help explain how complex insect societies evolve and how social and hormonal signals interact to shape colony structure.
Nathan Derstine, postdoctoral scholar in the College of Agricultural Sciences at Penn State, and Priscila Santos, postdoctoral fellow at Utah State University, were also co-authors on the study.
The U.S. National Science Foundation helped support this research.
