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Master Naturalists: Bumblebees work hard in our gardens

By By Paul and Mary Meredith
July 25, 2013 at 2:25 a.m.

Bumblebees like this one are primarily pollen-eaters, although they will drink nectar.  The yellow blob on its hind-leg is hundreds of pollen grains collected using a unique technique.  Bumblebees grab flowers that store their pollen in long, narrow tubes (anthers) and violently shake the flower - causing the pollen to shoot out the tubes and collect on the fine hairs on the bumblebee's body.  Moving on to another similar flower, the bumblebee will pollinate it.  This special form of cross-pollination is called sonication or buzz-pollination.  That's because a buzzing sound is made as the flower is shaken.  Flowers of buzz-pollinated plant species can only be pollinated by bumblebees.

Living in South Georgia in the late 70s, we grew tomatoes, peppers and eggplant, among other things. We also had a fence line of blueberry bushes.

All were required pollination by insects to bear fruit, most commonly by big, buzzing bumblebees - not honey bees, wasps or butterflies. Until last week, I never knew why that was or the significance of the buzzing I heard.

Pollination basics

Grasses and most trees produce massive amounts of pollen. They use wind-pollination, or anemophily.

Most fruits, vegetables, and ornamental flowers evolved later, when insects had evolved. Those plants produce less pollen, distributing it efficiently with the help of insects moving from flower to flower. This is called cross-pollination, or syngamy. Plants pay the insect by providing food to nourish them and their young.

Plants' most common cross-pollination strategy is to place nutrients at the base of their flowers, forcing insects to get pollen on themselves as they pass the plants' anthers to drink nectar. All honeybees do this, and they're messy - enough pollen stays on their bodies to pollinate efficiently.

But about eight percent of all flowering plants, including some very important ones, produce just pollen. Those plants evolved tiny, smooth pollen grains, stored in long, narrow, tubular "anthers" that only pollen-eating bumblebees can extract. How?

They use a technique, called buzz-pollination, or sonication. A bumblebee landing on a flower clutches the flower tightly. Contracting its flight muscles rapidly, hundreds of times a minute, vibrating the flower, it dislodges pollen, shooting it out onto the bumblebee. Hairs on the bee's body catch the pollen.

The bee grooms itself and puts most of the pollen in sticky cups on its hind legs for transportation to its solitary nest and young. Some pollen remains, supporting pollination.

How strong is the vibration? Biologists have measured the force; it can be as much as 30 times the force of gravity. That's one strong bee! And, the vibration produces a characteristic buzzing sound - about middle C on a piano - unique to a feeding bumblebee. That's why bumblebees buzz.

The second part of the mystery

Two groups of plants pollinate via sonication: Solanaceae (the nightshade family); and parts of the genus Vaccinium, commonly bush-berries. Why are bumblebees found on tomato, pepper, tomatillo and eggplant blossoms?

And why on our neighbors' potato vines when we were in school in Bloomington, Ind.?

All those plants originated in South America, and are in the nightshade family. And, yes, blueberries are in the genus Vaccinium along with cranberries and a number of other edible bush-berries - including indigenous wolfberry, a primary source of nutrition for wintering whooping cranes on the Aransas Wildlife Refuge.

There are also showy and beneficial buzz-pollinated plants. Common in Victoria-area yards, petunias, datura (Angel's Trumpet), decorative peppers and potato vines are all buzz-pollinators, although some hybrids self-pollinate.

Nicotiana species (tobaccos) are buzz-pollinated - along with Coffea arabica (coffee), a buzz-pollinated bush-berry. Plus many nightshade species produce alkaloids, which can be poisonous or have medicinal properties.

References and Sources:

"What's the 'buzz' about? The ecology and evolutionary significance of buzz-pollination", Current Opinion in Plant Biology, Vol 16, No 3, June 8, 2013,

Paul A De Luca1 and Mario Vallejo-Marn2

1 School of Chemistry, Environmental and Life Sciences, The College of The Bahamas, Oakes Field Campus, P.O. Box N-4912, Nassau, Bahamas

2 Biological and Environmental Sciences, School of Natural Sciences, University of Stirling, Stirling FK9 4LA, United Kingdom

"Unraveling the Pollinating Secrets of a Bee's Buzz", Carl Zimmer, NY Times, July 11, 2013

plantdex.com/index.php/planting-guides/plant-management/201-plant-pollination

solgenomics.net/about/about_solanaceae.pl

"Plants of Texas Rangeland," essmextension.tamu.edu/plants/plant/wolfberry/

Paul and Mary Meredith are master naturalists. Contact them at paulmary0211@sbcglobal.net.

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