Tom's Blog

Saturday, June 23, 2012

Butterfly milkweed at its peak

Butterfly milkweed (Asclepias tuberosa) is probably the most "showy" of the milkweeds, and is a species that invariably attracts attention in prairies and open savannas. Probably because of the early spring and unusual summer weather, this is a fantastic butterfly milkweed year.

Black Earth Rettenmund Prairie has probably the best display of butterfly milkweeds in southern Wisconsin and right now it is at its peak. Thursday evening Kathie and I led our annual field trip to this prairie, and although it had been advertised as a wood lily trip, it was a butterfly milkweed trip instead. I didn't count the number of plants, but there were certainly well over 1000. The "saddle" area in the middle  (which had been burned) had the best display, although there were great populations all over. You could see the color even driving by on County F at 55 mph! (The lead plant and coreopsis displays are also great!)

There is quite a lot of color variability in butterfly milkweed, ranging from yellow through orange to dark reddish orange. Many years ago, Robert Woodson, the world expert on milkweeds, did  a detailed study of color variation, using what was then a piece of high-tech lab equipment, a Cary recording spectrophotometer. (Woodson, Robert E. 1964. The geography of flower color in butterflyweed. Evolution 18: 143-163.)

Flower color here derives from two chemicals, carotenoid, which is yellow, and anthocyanin, which is red. The carotenoid concentration remains relatively constant whereas the anthocyanin color is superimposed on this and varies considerably. In yellow plants the anthocyanin is low or absent and as anthocyanin is added the color gets progressively redder.

On a motor trip from Kansas to coastal Virginia, Woodson collected more than 50 separate populations and found striking geographical variations. Minnesota, Oklahoma, and Kansas, for instance, had colonies which were all yellow, whereas in Missouri, Illinois, and Iowa orange-red predominated.

Although most of the butterflyweed at Black Earth Rettenmund is rich orange red, there are distinct color variations. Also, Kathie has found a pure yellow population in a planted prairie at Pleasant Valley Conservancy.


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