LOGAN, Utah, USA — From fundamental biological units as tiny as genes to complex societies, ecological systems rely on cooperation. All manner of organisms can benefit from working together to survive in a dog-eat-dog world.
"Mutualism is a common example of interspecies cooperation," says Utah State University ecologist Valerie Martin. "Scientists have long studied mutualisms between plants and pollinators, but understanding their origin and maintenance remains a challenge. Curiously, exploitative behaviors — including cheating — are rampant among mutualists and we're trying to understand why."
Martin, a doctoral candidate in USU's Department of Biology and the USU Ecology Center , along with her faculty mentor Robert Schaeffer, are studying nectar-robbing behavior in bumble bees through support from the National Science Foundation. They report findings in an online, preprint article posted Feb. 6 in the journal iScience.
Nectar robbers are termed as such, she says, because they take nectar from flowers without providing the service of pollination in return. Rather than extracting nectar from a flower's natural opening, which causes the bees to come in contact with the flower's reproductive structures and enables them to pollinate other flowers, the bee saves its time and effort by sipping nectar from a more easily accessible hole in floral tissue.
Martin says pollinators engage in two main types of nectar-robber foraging. One is called primary robbing, in which the floral larcenist creates its own hole in the flower. The other type, known as secondary robbing, involves making use of an already-created hole.
"Sometimes, secondary robbers don't have long enough tongues to avail themselves of a meal from a flower with deep parts, such as tubular corollas," she says. "A convenient hole is a not-to-be missed foraging opportunity."
Martin is among researchers who suspect other organisms beyond plants and pollinators may be aiding and abetting pollinators in nectar-robbing behavior.
"We've been testing the hypothesis that nectar-inhabiting microbes — namely yeast — are mediating larcenous behavior by bumble bees as they visit flowers," she says. "We suspect volatile organic compounds from yeast may be attracting pollinators, especially secondary robbers, to accessible feeding opportunities.
To test the hypothesis, Martin conducted research at the nonprofit Rocky Mountain Biological Laboratory near Crested Butte in Colorado. Known by its acronym "RMBL" — pronounced "rumble" — the nearly century-old high-altitude biological field station provides an ideal setting for pollination biology research.
"RMBL is an undisturbed mountain setting with a diverse range of native wildflowers, as well as expansive meadows teeming with native pollinators that haven't been infiltrated by invasive species," Martin says. "In addition, RMBL has decades of data to support our research, along with staff researchers with valuable expertise. Their staff was very helpful in advising us on how to set up our study."
Back in their Logan lab at USU, Martin and Schaeffer designed and constructed Y-tube olfactometer arrays to test bee scent preferences.
"We assembled source colonies of bumble bees from the bee rearing room at the USDA-ARS Pollinating Insects Research Unit in Logan and divided these bees into two groups, supplied with either sterile nectar made in the lab or the same nectar fermented for three days by four yeast species," Martin says. "Through these experiments, we conditioned the bees with either non-fermented or fermented nectar as food sources, then tested how that conditioning affects their smell and taste preferences for food outside the nest."
The researchers concluded yeast microbes do indeed attract robbers, but the robbers' presence may not be all bad.
"Though nectar robbers may cheat the flowers from which they're stealing nectar of pollination, their larceny may ultimately benefit the ecosystem," Martin says. "Bumble bees require a lot of nutrition to fuel their very fast metabolism — that is, to survive. By surviving, they live to pollinate other plant species and contribute to a thriving ecosystem."
