Honeybees evolved from solitary bees and hornets evolved from wasps. Honey bees and hornets are social insects.

Many social insects have an age-specific behavior in common: when they emerge from the pupa, workers begin their around-the-clock tasks within the dark nest by taking care of the brood. They then move quickly through a series of cyclic jobs that move them away gradually from the center of the nest towards the outer areas as they age, eventually becoming foragers which takes them exclusively into daylight near the end of their lives.

Researchers have found evidence in social insects that the shift from around-the-clock to rhythmic tasks seems to be driven by a slower maturation of the endogenous—or internal—circadian clock of social honeybees in comparison to solitary bees. This does not occur in solitary insects.

Their research indicates that the maturation of the circadian clock is delayed in social honeybees as compared to solitary mason bees. Both the male and female solitary red mason bee Osmia bicornis emerge with a fully functional, mature circadian clock. This is shown by their 24-h movement rhythm and the activity of their brain cells that produce the “pacemaker” protein Pigment-Dispersing Factor (PDF). Females of every age forage for food to feed their offspring.

This 2:59-minute video by OPALexplorenature introduces you to the red mason bee nesting:



The study was conducted by Dr. Katharina Beer, a postdoctoral scientist at the Department of Animal Ecology and Tropical Biology of the Julius Maximilian Universitaet Wuerzburg, Germany, along with Professor Charlotte Helfrich-Foerster, the Chair of Neurobiology and Genetics at Julius Maximilian University, and is published in Frontiers in Cell and Developmental Biology, an open-access journal.  

They predict that rapid maturation of the circadian clock is the ancestral condition and is found throughout the solitary bees and wasps, which all need to forage and care for the brood throughout their life spans. In contrast, a delay in maturation may have evolved secondarily in social species to enable age-related behavioral shifts from acyclic brood care to daily foraging.

The red mason bee, unlike social insects, has no infertile worker caste. The females overwinter in their nests and emerge in spring to mate and then build new nests in hollow stems. They forage on plenty of plant varieties for nectar and pollen, and they store it in a row of sealed cells, laying one egg per cell.

To register the rhythm of each bee, researchers collected 40 newly emerged female bees from isolated brood combs of honeybees (Apis mellifera) and 56 females and 31 males from isolated pupal cocoons of Osmia bicornis and placed them all inside separate tubes in a Locomotor Activity Monitor system where infrared beams cross at the center of each tube and are interrupted when bees move around.

The results are that no honeybees showed spontaneous ca. 24-h rhythm right after emergence from the pupa no matter how much contact there was with the natal colony. In contrast, 88% of the Osmia bicornis females and males did so and those that did not died young. By the time honeybees reach two days old, they develop a pronounced ca. 24-h rhythm.

Beer and Helfrich-Foerster conclude that solitary Osmia bicornis bees, but not social honeybees, emerge with a functional endogenous circadian clock.

The neural basis for this difference was examined by the researchers, who used immunohistochemistry to compare the maturation of the neurons that synthetize PDF, in the two species. A cluster of specialized cells called the lateral ventral neurons in the brains of insects, functions as a circadian pacemaker that secretes bursts of PDF, a so-called neuromodulator which affects the activity of the central nervous system. By staining dissected, fixed brains with two antibodies, the first recognizing PDF and the second, labeled with a fluorescent tag, binding to the first, Beer and Helfrich-Förster counted the PDF-producing neurons in each brain hemisphere at ages between pre-emergence up to 4 weeks later.

The authors conclude that the circadian pacemaker is fully mature in newly emerged Osmia bicornis but needs to develop to become fully active in honeybees, explaining why young honeybees don't yet show a circadian rhythm. This delay in maturation is likely an evolutionary adaptation to sociality, where young bees need to engage in around-the-clock care of their immature siblings.

Similar studies will be conducted on other social insects like ants, which have various forms of social behavior: some display age-related behavior like in honeybees, while others don't. 

Osmia bicornis is a highly valued pollinator worldwide so understanding its natural rhythms is very important.