When Is A Pollinator Not A Pollinator? Tales From An Insect Crime Scene...

Rebecca E. Irwin and Alison K. Brody of the University of Vermont have been studying these bandits, known as nectar-robbing bumble bees (Bombus occidentalis) at the Rocky Mountain Biological Laboratory in Colorado. A study published in the July issue of Ecology highlights their research.

Past studies have shown that nectar robbing may in fact have a positive effect on the reproductive success of a plant. Some robbers can unknowingly aid in a plant's pollination when they brush up against floral reproductive structures. In this study, the researchers wanted to know if nectar robbing was detrimental to the scarlet gilia (Ipomopsis aggregata).

To study the bee's effects on the gilia, the researchers first studied the method by which the robber steals nectar from the plant's blossoms. The bee uses its spiky, toothed mouthparts to chew a hole through the side of the corolla, the petals that surround the inner parts of the flower. It then sucks the nectar out of this hole through a long, snout-like proboscis.

While this method provides ample nectar for the bee, there is none left for other winged creatures, such as the hummingbirds which migrate through the region.

This larceny also fails to pollinate the plant, a process that would likely occur after a visit from a hummingbird. Pollination in the gilia occurs only through interplant pollen transfer. For a plant to be successfully pollinated, the pollen of one plant must be transferred to the stigma of another, where it can fertilize the ovule and form seeds.

Nectar robbing, therefore, has the potential to be highly damaging to the plant's reproductive success. Since individual gilia plants bloom only once, estimates of lifetime reproductive success can be measured in a single season.

The researchers measured the rate of pollen transfer between the scarlet gilia plants by placing dye particles on flowers to imitate pollen. The number of dye particles deposited on flowers were compared in plants with low and high robbing rates. This was associated with the amount of pollen transferred by pollinators.

The researchers found that highly-robbed flowers donated and received fewer dye particles. This meant that less pollen transfer was occurring among those plants which were visited often by nectar robbers.

"The most probable explanation for the reduced fitness of nectar-robbed scarlet gilia is that these plants attract less pollinators," says Irwin. "Hummingbirds tend to avoid plants which are highly robbed, and visit less flowers on those plants. Our study shows that nectar-robbing does decrease reproductive success in the scarlet gilia, further research will elucidate the effect of floral larceny on the evolution of floral traits."

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