Bumblebees Under Glass

I like flowers. The endless variety of form and color are a continuing source of amazement and contemplation. Not surprisingly, I also like to photograph flowers, and I make frequent trips to find new ones to photograph. One of the best places to find flowers, at all times of year, is a greenhouse - one of my favorite places to be.

There are lots of things to see in greenhouses besides flowers. I've seen birds, I've seen moths, and I've seen pillbugs. One of the more surprising things seen during a recent greenhouse visit was a bumblebee. That bee seemed to be touring the various rooms of the greenhouse just like me, looking for new and tempting blossoms.

I assumed the bumblebee had blundered in to the greenhouse accidentally, probably through one of the open vents in the roof. I was not aware at the time that this bumblebee may well have been bred especially for greenhouse duty, and she may have been released in there intentionally.

Why would a greenhouse owner be willing to pay for commercially reared bumblebees to be released 'under glass?' If you are interested in harvesting seeds from exotic plants, you would probably want some bumblebee pollinators around. But that research interest doesn't seem like enough to support a whole bumblebee rearing industry. Only another industry could do that.

In the case of bumblebees that industry is the commercial growing of 'hothouse' tomatoes. Tomatoes have an unusual flower. The petals are fused together into an urn-shaped tube that largely encloses the reproductive structures. The pollen grains are enclosed by the walls of the anthers and can only escape through tiny pores. Now these flowers are self-fertile. The flower can be fertilized by its own pollen, resulting in the growth of a tomato fruit.

Right: Tomato blossoms - a 'difficult' flower.

But the peculiar structure of the tomato flower means that fertilization can only happen if the blossom is jostled, shaking the pollen grains out of the anthers so they can fall on the stigma. Outdoors, a moderate breeze is enough to shake up the tomato blossoms, producing the tomatoes that go to market. But in a greenhouse there is no wind, and that is where bumblebees take over.

The tomato flower does not have much nectar, but it is enough to attract a hungry bumblebee. The bee lands on the flower and proceeds to vibrate its flight muscles, at the same time producing a loud buzz. The vibrations are transmitted to the flower and shake loose the pollen. This 'buzz pollination' is something honey bees cannot do.

So bumblebees have a special talent. Their skill with buzzing 'difficult' flowers has made commercially-reared bumblebees an important factor in the production of the greenhouse-grown tomatoes that eventually show up in our salads and pizza sauce.

[Photo by Niek Willems, CC license]

Big Bees and Little Bees

In the bee world, bumblebees are really big. A large queen bumblebee of a relatively large species, like Britain's Buff-tailed Bumblebee (Bombus terrestris) may be up to 22 mm (a little less than an inch) long. Compare that to one of the tiniest bees, the solitary bee, Quasihesma, from Australia, less than 2 mm (around 1/16 inch) in length. This tiny bee could easily sit on the head of a pin.

But bumblebees are by no means the largest bee. In North America, the common Large Carpenter Bee (Xylocopa virginiana) is still larger on average, with a maximum length of 23 mm (just under an inch). These carpenter bees look a lot like bumblebees, and I often have to look carefully to see the metallic-blue, shiny abdomen that distinguishes this species from the furry abdomen of bumblebees.

If you are bee watching in the jungles of Indonesia, you may see the rare Giant Resin-bee, (Megachile pluto), a full 39 mm in length (about an inch and a half). So the largest bee is about 20 times larger than the smallest one, with bumblebees coming out somewhere in the middle.

A large bumblebee queen has to be big, because to keep order in the colony she may have to be tough. Her worker daughters are not always the meek handmaidens to the throne that their station demands. Some of the larger workers may try to take over egg laying, although their eggs can only produce males. Then the queen will have to bully them back to their normal duties.

Bumblebee queens may keep their workers under control, but they don't win all battles in the nest. Cuckoo bumblebee females tend to be stronger and more heavily armored than the queens of colonies they parasitise. When the cuckoo enters the nest a violent fight with the queen ensues. Usually the cuckoo female is the winner and the queen dies.

Size is relative, and bigger isn't better, just different. Big bees are large simply because that size suits them to whatever role they play in the environment.

The Common Eastern Bumblebee - Impatient for Domination

The first person I spoke to about bumblebees told me that whatever I might see, it would probably be Bombus impatiens, The Common Eastern (or Impatient) Bumblebee (CEB). That seemed hopeful. Gosh, it shouldn't be any trouble at all to know what species I was observing.

Turns out things are not quite so simple as all that. There are actually as many as 25 species of bumblebees occurring in eastern North America. You can never be certain of the species you have unless you become familiar with the identifying features of each of them.

But my informant was certainly right about the pervasive presence of the Common Eastern Bumblebee. The dominant nature of this species in eastern bumblebee communities has been reinforced once again in collections made during 2009 by the USGS Native Bee Inventory and Monitoring Lab .

The data made available by bee biologist, Sam Droege, show that of almost 600 specimens collected in 10 eastern states plus the District of Columbia, 65% of the total were the CEB. In those states represented by more than a handful of specimens, Bombus impatiens constituted no less than 64% and in one instance as much as 77% of the total. This species is certainly the most commonly encountered in the east.

Fortunately the CEB is relatively easy to identify too, at least for females, the queens and workers. They are characterized by yellow pile that covers most of the upper surface of the thorax, often with a black spot in the middle. And they have just a single band of yellow pile across the upper surface of the first visible abdominal segment. The rest of the abdomen is covered with black, furry pile. Unlike many other bumblebee species, there seem to be few different color forms or morphs to confuse the issue.

All in all, a most convenient species on which to begin biological observations - common and easily identified. So be on the lookout this summer for the Common Eastern Bumblebee. It will likely be the first one you see.

[Photo by D. Barr]