Last week, I spent a lot of time watching two species of bumble bees on the few blossoms of Rosa rugosa that were left. They seemed to be in a frenzy inside the blossom. In the past when I have seen this, it looked like they were rolling in the pollen in ecstasy, much like our dogs roll happily in vile things in the woods. But on this day, I saw that their high speed movement had deceived me. They seem to be gathering pollen, but in greater quantities than they would individually need.
R. rugosa does not have nectar so it may not be particularly enticing to insects. It is self-pollinating, but it needs cross-pollination by insects to set fruit, which are the rose hips with which we are familiar. Rugosa is also called the beach rose. It is not native, but is extensively used to prevent erosion on beaches and dunes in the eastern United States. It creates many nostalgic memories for those of us who grew up on the Atlantic shoreline or visiting it often.
Pollen is the sperm of flowers. It is on the anthers. The anthers may have the pollen on the outside, easily available to insects, or it may be trapped inside the anther. Bumbles eat pollen as adults, but most importantly, they feed it to the bee larvae in the hive. Rose pollen is highly favored by bees because it's rich in nutrients. But the pollen in R. rugosa is trapped inside the anthers, so the bumbles need to get that pollen outside of the anthers so they can collect it for the baby bees.
Rugosa, as a genus, can be pollinated in one of or more of three different ways and all three methods can be used with R. rugosa:
1. Automatic or insect-mediated selfing within one flower (autogamy)
2. Pollen transfer from flower to flower on the same plant (geitenogamy)
3. Cross-pollination (xenogamy) results when pollen is transferred from the anthers of one plant to the stigmas of the flowers of another.
When you see a bumble inside a blossom, making a loud buzz sound, perhaps even "rolling around," it is collecting the pollen from inside the anthers. The bee (only female bumbles can collect pollen) uses her flight muscles — not to fly, but to vibrate her entire body, and the vibrations release the pollen. While her body vibrates rapidly, creating that buzzing, she bites the tip of the anther to release the pollen. After the release, the bee collects it in the sacks on her legs. This entire process is called buzz pollination.
Because the activity in my videos, below, happens so fast, I have added this slide show of still photos of the activity at one R. rugosa blossom.
Buzz pollination is also required for tomatoes, potatoes, and blueberries (all New World foods). And honeybees cannot do it, only our native bumble bees can. JF says that she also sees this activity on mullein and hydrangea.
In the videos below, that I made as part of my practice of making nature videos, you will see Common Eastern Bumble Bees (Bombus impatiens) and one Tricolored Bumble Bee (Bombus ternarius) buzz collecting pollen from R. rugosa. The tricolored bee has the orange band. Unfortunately, I deleted the sound. The buzzing is much louder than you normally hear, and it reminded me of angry bees. But they were not angry, they were simply in "overdrive."
The bees, though different species, did not seem to mind sharing a blossom with others. One bumble here or there would run off (especially the Common Eastern Bumble). On the other hand, it also did not look as if they enjoyed each other's company. Sometimes there was a slight tussle when another bee entered the flower. Sometimes there was not.
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