These sweat bees (genus Megalopta) are unique in that they forage in the evening and at dawn, well outside the normal foraging times of other day-flying bees. Here, a sweat bee arrives at a balsa-tree flower under cover of darkness. The bee is about the size of a honeybee, and the flower is about as big as a coffee cup. (Photo credit: Christian Ziegler)

By Leslie Mertz

Adam Smith, Ph.D., has been studying a certain group of very unusual bees in Panama for 15 years, but he had never actually been able to witness a key part of their behavior: their foraging forays. That’s because the bees get their pollen and nectar from flowers that not only bloom at night but also sit high in the rain forest canopy. “I always thought, what a shame that I can’t study it because they’re all up in these tall treetops,” says Smith, assistant professor in the Department of Biological Sciences at George Washington University.

Leslie Mertz

But then the unexpected happened.

Scientist-turned-photographer Christian Ziegler got funding from National Geographic to take photos of the monkeys, birds, and other animals residing in the Panamanian rain forest canopy. To do it, he built towering scaffolding up into the very trees where Smith’s bees foraged. Smith recalls, “He asked if we wanted to film some of our bees up there when he wasn’t using the towers himself, and I said, ‘Of course!’ I had studied these insects at their nests for years, and now finally we could go up and see what’s going on at the flowers.”

Christian Ziegler, a tropical biologist and photojournalist for National Geographic magazine, snaps a photo of the Balsa flower and some of the bees that visit the blossom during the day. (Photo credit: Adam Smith)

Sweat bees do their foraging high in the rainforest treetops. After years studying the bees, Adam Smith, Ph.D., was finally able to witness their foraging, thanks to scaffolding towers that were constructed for a photographer. (Photo credit: Adam Smith)

A little ingenuity was involved in getting video of the sweat bees at the flowers. Here, infrared (night vision) video cameras are lashed to the scaffold. (Photo credit: Adam Smith)

Adam Smith, Ph.D., (left) and then-graduate student Margarita Lopez (an assistant professor at Penn State) open up a sweat-bee nest in the forest surrounding the Barro Colorado Island field station in Panama. The bees dig tunnels into dead sticks suspended in bushes in the tropical-forest understory. (Photo credit: Marcos Guerra, Smithsonian Tropical Research Institute.)

Scientist-turned-photographer Christian Ziegler gives a triumphant smile atop the scaffolding tower. (Photo credit: Adam Smith)

Smith’s long-term fascination with the bees, which are two species of sweat bees in the genus Megalopta, stems from his interest in animal behavior and, particularly, social behavior and cooperation. “As an undergraduate student, I studied birds and bird song, but if you’re interested in social cooperation, the bees, wasps and ants have so much to offer,” he says. After switching focus to entomology, he began to ponder the costs and benefits of living in a group versus living alone, and that led him to a few species of bees and wasps that do both. The Megalopta species in Central America were among that number.

While the sweat bees’ extraordinary social flexibility is Smith’s primary interest, he has always been intrigued by one other quirk. “They have this other thing that is even weirder for bees: They fly when it’s dark. Very few bees do that, because bee vision is adapted to daylight, and of course flowers are typically day-active,” he says. Other researchers studied their vision and found that the bees still need at least some light to see, and Smith’s own work videotaping the nests showed that they do all their foraging during two approximately hour-long spurts, one around sunset and another around dawn. The question was, why?

“Here we had these sweat bees sitting in their nests 22 hours a day. I could understand why they can’t fly in the middle of the night—they can’t see—but why not start a little bit earlier in the afternoon or go a little bit later in the morning so they can get in some more foraging?” he says. “I just thought, Well, they’re lazy. That was my working hypothesis,” he laughs.

Smith gained a new appreciation for the bees once he climbed Ziegler’s towers and got a good look at what was happening. It turned out that the sweat bees forage for a short period around sunset and sunrise for a couple of reasons, which he describes in a new article in the Journal of Insect Science. First and foremost, he says, they do it to avoid bullying from other more aggressive bees that live in large colonies and visit the flowers in broad daylight. In fact, he says, they saw absolutely no overlap in the sweat bees with other bees at the flowers. “This is highly suggestive of the idea that the bees are avoiding what we call ‘interference competition,’ and instead are making a few very efficient trips to get all their foraging done quickly when no one else is around to bother them.”

The short foraging time also makes sense from a parenting standpoint. When adult sweat bees are away from the nest, he explains, ants will enter and eat the young. “That means that there should be a very strong selection pressure to getting the foraging done as quickly as possible so they can spend most of their time at their nests to guard against predators.” The bees, he concludes, are not lazy after all. Rather, they are very efficient foragers and excellent parents.

That mystery solved, Smith is back to his primary studies of social versus solitary behaviors, but he will always remember his time in the treetops. “It was a ton of fun. Just being up on these towers at night and hearing the night insects come out, or going up at early in the morning, watching the sun rise and seeing the birds and monkeys wake up, it is just really a special place to be,” he says. “And to get up in the trees and observe these insects in a new way, it was just wonderful.”

Leslie Mertz, Ph.D., teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.