Graduate student Dara Satterfield caused quite a flutter recently when she was featured in the New York Times as the co-author of a study looking at how Tropical milkweed, Asclepias curassavica, may be effecting the health of Monarch butterflies and their Pan-American migration.  Her dissertation focuses on the relationship between migration and infectious disease in wildlife, with Monarch butterflies as her species focus.

Dara Satterfield

Dara Satterfield, PhD candidate at the Odum School of Ecology at the University of Georgia.. Photo by Monika Maeckle

In the article headlined For the Monarch Butterfly, a Long Road Back, and promoted heavily online as “Monarch Butterflies:  Loved to Death?” science journalist Liza Gross explored the pros and cons of planting Tropical milkweed.   To read our original story on this topic, check out Tropical Milkweed:  To plant it or not, it’s not a simple question.
Satterfield, a PhD candidate at the Odum School of Ecology at the University of Georgia, and other scientists speculate that Tropical milkweed, the most widely available but technically nonnative milkweed and favorite host plant of the Monarch butterfly, may be damaging the Monarchs’ abilities to stay healthy, on track and make their way to Mexico.
“She and her graduate adviser, Sonia Altizer, a disease ecologist at Georgia, fear that well-meaning efforts by butterfly lovers may be contributing to the monarch’s plight,” said the article.
I caught up with Satterfield recently to ask questions that have arisen since the article posted on November 17.   She expressed concern that the NY Times article might have confused some readers–and no doubt the issue is confusing and complex.   Hopefully the Q & A below will clarify matters a bit.
Q: I’ve talked to several scientists that insist that Tropical milkweed is the plant on which Monarchs evolved. Do you agree with that?

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PhD candidate Dara Satterfield doing field work on Tropical milkweed and the Monarch butterfly migration. Photo courtesy Dara Satterfield

A:  Good question. From what I understand, the historically held view was that Monarchs evolved from a tropical ancestor from Central or South America, and so some scientists have said they must have used Tropical milkweed and other exotic milkweed species early in their speciation.
New evidence suggests a different story. The recent Nature paper examining Monarch genetics revealed that, actually, Monarchs appear to have originated in North America (and would have evolved on native North American milkweed species) and the other Monarch populations in Central America, South America, the Pacific, etc. (some of which would use Tropical milkweed) came from the North American population.
Q. You have said that Monarchs are much more likely to be sick in places where Tropical milkweed grows year-round–but is it really Tropical milkweed (Asclepias curassavica) that is the problem? If Common milkweed (Asclepias syriaca) or Swamp milkweed (Asclepias Incarnata) also survived a winter and were available, would the same tendency apply?
A. You are correct, I think. The same disease problem would probably occur with any milkweed species that grew year-round in warm areas and was attractive to Monarchs. It just happens that Tropical milkweed is the species that does stick around. We don’t think Tropical milkweed itself is bad; it’s the year-round growth that is harmful because it promotes disease.  Also, I’d just like to add that we would not even understand this problem without the help of dedicated volunteers and citizen scientists who share observations and collect data. Much of what we know about Monarch ecology can be attributed to the help of citizen scientists.
NOTE from Texas Butterfly Ranch:   Thus, best practice suggests slashing all milkweeds to the ground in late fall if they do not die back from freeze.  This prevents OE spores from building up and spreading disease.

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Satterfield in the lab, checking for OE spores. Larvae can acquire OE infections by eating parasite spores on milkweed leaves, left there by an infected butterfly (often, the larva’s mom). Courtesy photo

3. What is the purpose of a migration? If everything an insect needs to complete the life cycle is available locally, what interest is there for the insect to migrate?
For most migratory species, the purpose of migration is to track seasonal changes in climate or resources needed for survival and reproduction. Without human interference, migration as a strategy can often support large numbers of animals, because migratory animals may take advantage of the best resources–in different parts of the world at different times of the year (e.g., red knots that travel from the North Pole to the South Pole to experience summer in both hemispheres).

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Monarch caterpillar on Tropical milkweed. The larvae can pick up OE spores through contact with other creatures or from plants on which the spores rest. Courtesy photo.

But some migratory populations including birds, bats, fish, and hoofed animals are altering their migrations–shortening or halting their journeys–in response to human activities like barriers in their migratory pathways (e.g., dams), changes in climate, and human-provided foods. Examples of this abound (No Way Home, by David Wilcove). Of course some of these newly non-migratory animal populations will be just fine and learn to adapt to new circumstances, but others will not.
Consequences will include changes in infectious diseases, loss of ecosystem services associated with migration (e.g., nutrient transfer between ecosystems by salmon, control of insect populations by birds), and in some cases, species extinction.
For Monarchs specifically, their migration allows them to have a large population capacity. If Monarchs solely engaged in winter-breeding, rather than overwintering in Mexico, this strategy could likely only support a much smaller population. So we try to conserve the abundance of migration.
Of course, individual animals operate on an individual basis and do not make choices based on what is best for the population at large, so individual animals will often take advantage of resources that are available to them–for example, why go to Mexico when I have everything I need here?
The problem with that, in this case of year-round milkweed and year-round Monarch breeding, is extremely high levels of protozoan disease as well as risks of winter starvation (running out of Tropical milkweed) and freeze events that kill caterpillars. The concern is also that migratory Monarchs (or their offspring) might be exposed to parasite-contaminated milkweed in the spring.
All of that said, Dr. Chip Taylor is correct that the link between year-round milkweed and disease is by no means the largest threat to Monarchs.
However, given what we now know about this problem, we have the opportunity to reduce disease in Monarchs by keeping milkweed seasonal rather than available all year.
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