Painted cup adapts to pollinators, root hosts

The High Plains Trail, in the Open Space & Mountain Parks on the plains south and east of Boulder, has several wildflowers that are not found in the foothills or forests of the

Front Range. One that I had not seen elsewhere was a small plant with cream, green and pink flowers.

Castilleja sessiliflora is an Indian paintbrush, commonly referred to as prairie fire, Great Plains Indian paintbrush, downy Indian paintbrush or downy painted cup.

Most of the 200 species of Castilleja share a common flower form and are easily recognized as Indian paintbrush, but I did not recognize painted cup as a paintbrush.

Long, slender flowers extend from green bracts (modified leaves) that can be tinged with pink or red. Petals have fused to a vertical, inch-long corolla tube that curves at the top to an opening with a long upper lip and a shorter lower lip with three lobes.

Indian paintbrushes are facultative root parasites, with their roots grafting to the roots of grasses and forbs to take water, minerals and nutrients. A facultative root parasite can live and survive without hosts, but it grows faster, is more robust and lives longer if it can parasitize neighbors.

But the list of host species is not complete for painted cup. A study in a dry lime prairie or juniper savanna in Wisconsin found that painted cup was an obligate root parasite on two junipers, eastern redcedar, Juniperus virginiana, and oldfield juniper, J. communis. In that environment, painted cup was always clustered around one or more junipers and not extending beyond the drip line, or the outer circumference of the crown.

This pattern of distribution indicates that painted cup is not able to survive without one of these two juniper hosts. However, in Minnesota paintedcup is facultatively parasitic on a variety of grasses, such as hairy grama and June grass as well as other

wildflowers. The painted cup that I found on the High Plains Trail were not close to any junipers.

The same study in Wisconsin identified pollinators visiting wildflowers, including downy painted cup. The only pollinator servicing painted cup was the yellow bumblebee, Bombus fervidus, known for its exceptionally long tongue. B. fervidus also has a narrow head, and it was believed that the combination of narrow head and long tongue were needed to access the nectaries at the bottom of the long corolla tube.

Observers documented that “the head of the bee plunges as far as possible into the flower” in its effort to reach the nectar. From these observations, one might conclude that only B. fervidus could pollinate painted cup.

A study of pollinator activity in Minnesota listed six bumblebees pollinating painted cup, three with long tongues and three with medium-length tongues. (Thanks to Carol Kearns for bee descriptions.)

Why is it that only a single long-tongued bee could pollinate painted cup in Wisconsin, but in Minnesota, bumblebees with either long or medium-length tongues pollinate the same species?

A group of biologists led by Dr. Krissa Skogen at the Chicago Botanic Garden has been studying downy cup across its range. They found that sphinx moths, which have very long tongues, are the most common pollinators at the eastern edge of downy cup’s geographic range.

They also noted that at the eastern edge of painted cup’s geographic range the corolla tubes were white and very long. But in the western portion of the range, floral tubes were light pink and shorter and wider. Furthermore, these flowers were pollinated by bees and sphinx moths with either medium or long tongues.

These observations are consistent with hypothesis that floral traits evolve to attract adequate and reliable service from the available community of pollinators.

It is evident that downy painted cup has evolved to different pollinator communities and different communities of root hosts. It is less evident but certainly true that this process continues today, as the communities of pollinators and potential root hosts respond to climate change and the ever-increasing impact of humans on natural habitats.