it’s all about the buzz

You’ve seen how busy bees gather pollen from some flowers — for example, they systematically crawl over the surface of Coneflowers and Black-eyed Susans, the tops of which are dotted with little ray flowers sticking their pollen up for grabs as the bee comes by.  Many different kinds of pollinators might walk around picking up pollen from these plants, so there is lots of competition for the pollen. But there is no guarantee that they will deposit any pollen on a nearby neighbor — they might just fly off to a completely different group of flowers instead.

A Common eastern bumblebee and a smaller green sweat bee competed for the pollen on some New England Aster flowers.

Other flowers present a nectar reward to the bee if it will kindly crawl into the tubular flower, brush against the anthers to receive pollen and then kindly deposit that pollen in another of the same type of flower they subsequently visit.  But who knows which flowers the bee might visit next — it might not be the same species at all.

A honeybee can crawl down the floral tube of the bee balm flowers to get at the nectar at the base, but the bee is not big enough to brush against the anthers (yellow-brown structures sticking out of the flower in the photo) to get a dusting of pollen.  Hummingbirds are a better fit for these flowers, and transfer the pollen from plant to plant very effectively.

Still other plants produce flowers that protect their pollen for just the right pollinator, one that specializes in picking up pollen from particular a species, and reliably deposits some of that pollen on a neighbor of the same species for some healthy cross fertilization.

Common eastern bumblebees are the chief pollinators of Spiderwort flowers in the spring.  The pollen in these flowers is encased (not free) in the anther, which opens from a pore at one end.

By locking the pollen up in a capsule, it is protected from just any random pollinator walking over the flower.  Shaking the flower might dislodge some of the pollen, but most pollinators can’t manage that.  Instead, bumblebees and some solitary bees grasp the anther capsule with their legs, or even mouthparts, and vibrate their wings at a very high (and audible) frequency — and pollen comes flowing out the pore at the end of the capsule, dusting the bee.  This technique is referred to as “buzz pollination”.

The video below illustrates the bumblebee action nicely:

“much of the food we eat owes its existence to the buzz of the bumblebee”

The “buzz” about bumblebees

Bumblebees are common on many different flowers.  They don’t seem particularly fussy about where they obtain nectar or pollen, and they don’t seem to be very aggressive toward one another. Take, for example, the following scene of two bumblebees sharing a patch of cup plant flowers.

On closer inspection, these seem to be different bumblebees.  Bumblebee #1 (right) has light blonde hairs covering his back and abdomen,

while Bumblebee #2 has black hairs on his abdomen.

I think the above bee (#2) is probably the Common Eastern Bumblebee (Bombus impatiens), which really is the most common bumblebee in the eastern half of the US, from Maine to Florida.  The western limit of its range is North Dakota and points south, but recently it has been introduced to California to assist with pollination of various fruit crops there.  It is widely adapted to urban and suburban as well as agricultural areas, and has unusually large colonies and a long active season, extending well into the cool fall.

Bumblebee #1 looks like the Lemon Cuckoo Bee (Bombus citrinus), which often parasitizes the nests of other bumblebee species, using them for refuge from weather and access to nectar and pollen stores.  This bee is also an eastern US resident, but primarily at more northern latitudes.  Apparently the bee is highly endothermic, generating its own heat with wing muscle contractions, but it is also very sensitive to falling temperatures, and retreats to the nest when it gets too cold.

Bumblebees can be more effective pollinators than honeybees for some crops, because they can fly at lower temperatures and at lower light intensities.  They promote pollen release from some plants by grabbing the anther near the pollen sac and vibrating their wings (“buzz pollination”).  This behavior is beneficial for cross pollination of tomatoes, peppers, cranberries, and blueberries.  Who knew?