By tagging individual bumblebees with microchips, biologists have gained insights into the daily life of a colony of bumblebees (Bombus impatiens) in unprecedented detail.
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The team found that while most bees are generalists collecting both pollen and nectar over the course of their lifetime, individual workers tend to specialize on one of the two during any given day, dedicating more than 90 percent of their foraging sorties to either pollen or nectar.
The observations also revealed that individual bumblebee workers differ vastly in terms of their foraging activity.
Just like their domesticated cousins, the honey bees, bumblebees play important roles as pollinators, thus helping in agriculture and fruit production. But despite the ecological services they provide, many aspects of their biology still remain a mystery.
By outfitting each bumblebee with a radio frequency identification, or RFID, tag—similar to the ones used to protect merchandise from shoplifters—the researchers were able to keep tabs on them at all times and log the data automatically instead of relying on human observations limited to certain times.
"The way these studies have typically been done requires a human observer sitting in front of a hive entrance and taking notes all day, and nobody wants to do that," says Avery Russell, the lead author of the study. Russell is a doctoral student in entomology and insect in the lab of Daniel Papaj, a professor in the University of Arizona's Department of Ecology and Evolutionary Biology.
"With the RFID chips, we can track every nectar and pollen collection trip made over each worker's lifespan and a portion of the colony's lifespan."
The researchers then used this data to determine how patterns of specialization on each food type differed at timescales of a day or over a lifetime. The results are published in the journal Scientific Reports.
Once a bumblebee queen has mated, she burrows into the ground and overwinters. The following spring, she emerges and starts a hive that lasts until the fall.
A typical bumblebee colony grows to about 75 workers, with about 40 to 50 going out and foraging on flowers for nectar and pollen. After the colony's growth phase, the colony produces unfertilized eggs that hatch into males.
The male bumblebees then disperse in search for other unmated queens to begin the cycle anew.
"Each individual bee only lives between two weeks to a month at the most," Russell says, "and even though they behave as generalists over their lifetime, our study showed that they tend to specialize on one food source over the course of a foraging day."
The researchers were surprised to find a vast difference in efficiency, with the most active foragers making 40 times the number of trips each day as the least active workers.
"Interestingly, when we studied the morphology between very active foragers and workers that barely leave the hive, we found that bees with more sensitive antennae foraged more," Russell said.
Similar variation has been in observed in honey bees and other eusocial species, where some workers are much more active than others, but no one had seen it to this extreme due to the limits of human observations.
"If you watch a bee only for an hour or so, you can't say what it will do over the course of a few days or over its whole life," Russell says.
"We don't yet know why, but it could be that workers that forage less do so because they aren't quite as skilled at foraging as others and make themselves useful by doing more around the hive."
To track the bees' behavior, the team superglues tiny RFID tags to the backs of the bees. Each tag weighs only 2 to 3 percent of the bee's weight.
A Y-tube connects the hive to two arenas, one that offers pollen and one that offers nectar. When a bee leaves the hive to forage, it can choose to go to the pollen chamber or the nectar chamber.
Two RFID readers mounted at the entrance keep track of the bees going in and out and help the researchers collect a wealth of data. ■