Who Pollinates the Flame Azalea?

By and large, one of the most endearing aspects of doing research in Southern Appalachia are the myriad Ericaceous species you inevitably encounter. Throughout the growing season, their flowers paint the mountainsides in a symphony of color. One of my favorite species to encounter is the flame azalea (Rhododendron calendulaceum).

This shrubby spectacle is a common occurrence where I work and its flowers, which range from bright yellows to deep orange and even red, put on a show that lasts a couple of weeks. It's not just me who enjoys the flowers either. Countless insects can be seen flitting to and from each blossom, sucking up rich reserves of nectar and pollen. It is interesting to watch a bee visit these flowers. Their outlandishly long anthers and style seem to be mostly out of reach for these smaller pollinators.

Bees attempting to grab some pollen look outlandishly clumsy in their attempts. What's more, small insects only seem to be able to get either nectar or pollen on any given visit. Rarely if ever do they make contact with the right floral parts that would result in effective pollination. Indeed, I am not the only person to have noticed this. Despite being visited by a wide array of insect species, only large butterflies seem capable to pollinating the flame azaleas stunning blooms.

The mechanism by which this happens is quite interesting. The reason small insects do not effectively pollinate these flowers has to do with the position of the anthers and style. Sticking far out from the center of the flower, they are too widely spaced to be contacted by small insect visitors. Instead, the only insects capable to transferring pollen from anthers to stigma are large butterflies. What is most strange about this relationship is that it all hinges on the size of the butterflies wings.

Only two species of butterfly, the eastern tiger swallowtail and the orange spangled fritillary, were observed to possess the right wing size and placement to achieve effective pollination for the flame azalea (though I suspect other larger species do so as well). This is quite unique as this is the only report of wing-mediated pollen transfer in northern temperate regions. The research team that discovered this noted that pollen transfer was greatest with the eastern tiger swallowtail, which is a voracious nectar hunter during the summer months.

Despite their popularity in pollinator gardens, butterflies are often considered poor pollinators. That being said, pollen transfer via wing surfaces has been a largely overlooked mechanism of pollination. Coupled with a handful of reports from tropical regions, this recent finding suggests that we must take a closer look at plant pollinator interactions, especially for plants that produce flowers with highly exerted anthers and stigmas. As the authors of the study put it, "transfer of pollen by butterfly wings may not be a rare event."

Photo Credit: [1]

Further Reading: [1]