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]