Sunday, August 25, 2013

The Pollination Trap

Bumble bees are so cool. One facet of their lives I find most interesting is their interaction with flowers in their role as pollinators. A bee covered in pollen grains is a magical sight to me, but a couple of summers ago I discovered that the floral kingdom can be endlessly inventive in creating methods of insect pollination.

A perennial I really enjoy in my garden is the July-blooming Asclepias tuberosa. A member of the milkweed family, it is also known by the common name butterfly weed. Its thousands of tiny, bright orange, nectar-filled flowers are a trifecta for attracting pollinators – hummingbirds, butterflies and bumble bees all love it. So I was kind of surprised to discover that asclepias has a darker side.

One day a few years ago I was observing a yellow-faced bumble bee (Bombus vosnesenskii) perched on a sneezeweed flower. I noticed the bee was acting strange - stumbling around while frantically trying to groom itself.

When I leaned in for a closer look I saw that its claws were covered with teardrop-shaped flakes of what I assumed to be nectar and pollen. I'd never seen anything like it. I tried to figure out where this weird-looking, sticky pollen was coming from but couldn’t fit the puzzle pieces together So I decided to take some pictures and a couple of videos and send them on to Carol Anne Kearns, a bumble bee expert who wrote the book The Natural History of Bumblebees.

I asked her, "I'm wondering if you have ever seen this before? I couldn't find anything online about it. It kind of looks like the bumble bee is wearing clown shoes. I was trying to figure out which flowers in my pollinator garden were causing this. I think it must be the butterfly weed, because that's what all the vosnesenskii bumble bees are focused on right now. They will nectar from the sneezeweed as well, but they also like to use it as a place to groom, since it's right next to the Asclepias."

Ms. Kearns kindly wrote me back saying: "Butterfly weed produces pollinia - sticky packages made up of many pollen grains, rather than individual pollen grains. That seems to be what is stuck on this bee!
See for a photo."

Pollinia, eh? I’d never heard of it. But now I wanted to find it in an asclepias flower. That turned out to be easier said than done. I went to my garden and harvested a few of the tiny flowers. I tore them apart but couldn’t find any pollinia. Where were they?

I decided to dissect a few more flowers, concentrating on the very base of the blossom. Finally, I found one

No wonder I had difficulty - they’re really tiny – no bigger than Franklin Roosevelt’s nose on a dime! They’re pretty cool looking though – like miniature wishbones.

According to the information that Ms. Kearns sent me, pollinium (plural pollinia) is a pollination strategy that only a few species of flowers utilize, asclepias and orchids among them. From what I’ve seen and read, it can be as sticky as velcro onto a bee’s legs and so resistant to release from the blossom that occasionally a bumble bee will pull its leg off in the struggle to free itself.

There is always something fascinating going on in the pollinator garden, and this year I will again be looking for bumble bees wearing clown shoes. But at least now I’ll understand what it is I’m seeing, and appreciate anew the complex and wondrous interaction between bees and flowers.

Other Resources:

Wiki on pollinium:

Beautiful photo of milkweed pollinium:

Showing how a pollinium is removed from a milkweed flower:

The effectiveness of different pollinators that come in contact with milkweed pollinia:

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