Milkweed Varies in Nutrition and Nice, Tasty Cardenolides
Milkweed Varies in Nutrition and Nice, Tasty Cardenolides
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Monarch butterflies must grow where they’re planted (or, rather, laid) but David De La Mater says if he were a monarch caterpillar with a choice, he’d pick a nice stand of milkweed in New England.
De La Mater is a master’s student in William & Mary’s Department of Biology. He is researching regional differences in milkweed and the implications of those differences on populations of monarch butterflies in eastern North America.
“My project seeks to look at the entire range of common milkweed,” he explained. The project builds on the field sampling of milkweed work initiated by his advisor, Assistant Professor Harmony Dalgleish.
“I want to kind of quantify the variation between populations, looking at demographic traits, how they grow,” De La Mater said. “I am also looking at defensive and functional traits.”
The milkweed-monarch connection
Common milkweed, Asclepias syriaca, is best known for its intimate, symbiotic relationship with one of America’s most charismatic insects, the monarch butterfly. Adult monarchs lay their eggs on milkweed. The caterpillars hatch and eat milkweed leaves, growing through various stages, known to scientists as instars. In the fullness of time, each caterpillar will curl up in a J-shape and spin itself into a chrysalis, from which the adult butterfly emerges, wings and all.
For a monarch — caterpillar or adult — milkweed matters. De La Mater points out that monarchs get not only nutrition from milkweed, but also defense against predators. The “milk” in milkweed is filled with compounds called cardenolides, toxic to almost all predators, but not a problem for monarchs.
The leaf tissue, food for developing monarch caterpillars, is rich in the toxic compounds, as well. In fact, monarchs are famous for their ability to sequester cardenolides, thereby making both caterpillar and adult unpalatable to predators.
But the amount of those defensive toxins varies across the common milkweed’s range, as does the nutritional value of the plant, as represented by a high nitrogen to carbon ratio.
“If I’m a monarch caterpillar, I’m going to look for a plant that’s high in those toxic compounds,” De La Mater said. “I also want a plant that has a lot of nitrogen, maybe not so much carbon.”
He said that studies of field across the range of common milkweed showed patterns: Plants in the east have a higher nitrogen to carbon ratio, while plants in the north show higher levels of toxic compounds.
“My hypothesis would be this: If I were a monarch caterpillar, I would enjoy eating the plants in the Northeast more than those in the Midwest,” he said. “So, yeah — milkweed from New England.”
De La Mater is conducting what is known as a common garden experiment. He brought milkweed plants dug up from all over the eastern half of the U.S. and installed them in pots in the greenhouse atop William & Mary’s Integrated Science Center. The idea, he said, is to see what happens to the nitrogen-carbon ratio and cardenolide level of the milkweed when they’re raised in a uniform environment.
He refers to the plants as “the guys,” and the guys display quite a bit of variation. They’re all specimens of Asclepias syriaca and they’ve been getting exactly the same TLC, but the variability is apparent at a glance. There are short plants, skinny plants, scraggly plants with yellowing leaves and sturdy plants that look capable of feeding an armada of caterpillars.
“It’s pretty neat. I like to step in here and imagine that this is a representation of the entire range of this species,” De La Mater said. Even in this common environment, you can see the difference in the species. I look at this collection and say there’s obviously variation across the entire range.”
De La Mater has been testing the plants for cardenolide levels and nitrogen-carbon ratio. The first tests, when the plants were collected in the field, showed quite a bit of variability, but after the plants had settled in the greenhouse for a while, something strange happened.
“After a year in the greenhouse, they all came to about the same level as to the compounds in their leaves, he said. “That was a very interesting finding. I expected that, but I didn’t expect the effect to be so drastic. It was pretty neat to see them all level out.”
The leveling out of the internal chemistry of the greenhouse milkweed was a good indication that the differences in nutritional and toxin components have environmental, rather than genetic differences.
“It has to matter for a monarch flying through the range, because that’s the main plant they’re eating,” De La Mater said. “Ninety percent of the eastern migrating monarchs feed on common milkweed.”
He explained that if you see a monarch in the eastern U.S. or Canada in the fall, it’s a member of the migrating or diapause generation. These butterflies go all the way to Mexico to overwinter on oyamel fir trees.
“The trees have these special properties that keep butterflies alive during the winter, allowing them to stay alive while not expending too much energy, he said. The overwintering monarchs come out of diapause — becoming sexually viable — in the spring and make a short hop to Texas, where they reproduce.
That second generation continues the northeastern migration. De La Mater said it takes three to five returning generations of monarchs to populate the range and produce the next Mexico-bound migratory generation.
He also added that there are two other populations of monarchs. There’s a group that migrates between the Rockies and wintering grounds on the coast of California. A population of the butterflies in southern Florida is nonmigratory.
The eastern monarch population is declining, and researchers cite a suite of reasons including habitat loss, pesticide use and the decline of milkweed. (The monarch populations of Florida and the West Coast are not in trouble.)
He uses high-performance liquid chromatography (HPLC) and spectroradiometry to analyze the different cardenolide compounds in the milkweed leaves. Recently, De La Mater has extended his studies to monarch caterpillars themselves. Using eggs obtained from a commercial supplier, De La Mater raises caterpillars in a controlled environment on leaves harvested from his greenhouse plants. Use of a growth chamber allows him to see how much good the caterpillars are getting out of the milkweed and to assemble regional comparisons.
“We want to capture all the frass (feces) and compare it against body-mass gain, to see how efficiently they are converting leaf tissue to body mass,” he explained.
He’s raised two sets of monarchs so far. He doesn’t have to wait on the turn of seasons: His control of light allows him to jump-start the development cycle of his monarch.
De La Mater’s work will help with the understanding of the problem by quantifying regional differences in the nutrition/toxin levels of milkweed.
“I know a lot of the variation has to do with environmental conditions — variation in climate, latitude, precipitation,” he said. “A lot of it has to do with where it’s growing. That’s why I wanted to do this, to see if there’s any variation of the plants without variability in environment.”
This article has been republished from materials provided by William & Mary Department of Biology. . Note: material may have been edited for length and content. For further information, please contact the cited source.