Wednesday, September 30, 2015

The monarch expert weighs in on tropical milkweed

Should we or should we not plant tropical milkweed (Asclepias curassavica)?  

Dr. Lincoln Brower, the world's foremost monarch butterfly expert, answered the question in his January 2014 essay, "On the wisdom or lack of wisdom in planting Asclepias curassavica outside of its normal range."

Asclepias curassavica is a tropical American milkweed and its natural distribution is Mexico, Central America, South America and the Caribbean (see Woodson 1954 reference below). It is very likely that curassavica has been intimately involved with the long term evolutionary history of the monarch butterfly in the Neotropics but NOT in the temperate zone. In that sense, allowing curassavica to reproduce in North America is encouraging an exotic weed. 

I have cultivated curassavica in a greenhouse at Amherst College in Massachusetts, in a large milkweed garden at the University of Florida in Gainesville, and at my former home near Gainesville. It is an ideal food plant of the monarch butterfly and is cultivated extensively to maintain commercial and experimental monarch cultures.  However:

The first problem with planting curassavica is that monarchs are highly attracted to it, lay their eggs and usually overwhelm the plants with caterpillars.

The second problem, and a more serious one, is that when monarchs are in their non-reproductive phase (gonads repressed in the fall), they will be almost irresistibly attracted to curassavica, remain near the plant, and come into reproductive condition. When this happens, as far as we know, monarchs lose their migratory urge.......and probably, as individuals, never get it back.

Evidence of this was dramatically demonstrated in Gainesville. I had about 50 well developed potted curassavica plants that I set out in my yard and another 100 or so that we planted in a garden at the University.  Fall migrants were highly attracted to the plants, laid eggs and there were so many caterpillars that I had to cull them.  At home where I kept a closer eye on them, the migratory monarchs produced an early fall generation and their offspring then produced a second generation.  The caterpillars formed their chrysalids by the dozens under the eaves of my house. By then it was mid- to late November, and the temperature cooled down, slowing development. Then a frost occurred and 100 percent were killed in the chrysalid stage. (Gainesville generally has two or three killing frosts each year.) I had this happen over several years.

I also visited an enormous county milkweed garden near Tampa, Florida, in the fall, and the monarchs had completely stripped hundreds of the curassavica plants. Since all native Asclepias die back in Florida, what could the butterflies then do?

Another problem with establishing what becomes a continuously breeding population of monarchs is that the incidence of the protozoan parasitic disease (Ophryocystis elektroscirrha) increases with time. If the diseased individuals then somehow end up breeding with or joining the overwintering clusters of monarchs, then the incidence of the disease will very likely rise with detrimental effects on the migratory monarch populations.

Professor Sonia Altizer at the University of Georgia, who is the world expert on OE, agrees with me.

I would limit curassavica to be used in inside demonstration projects, growing the plants in an enclosed area totally inaccessible to the wild monarchs.


Altizer, S.M., Oberhauser, K.O., and Geurts, K.A. 2004. Transmission of the protozoan parasite, Ophryocystis elektroscirrha, in monarch butterfly populations: implications for prevalence and population-level impacts. In: Oberhauser, K.S. and Solensky, M. (eds). The Monarch Butterfly: Biology and Conservation. Cornell University Press.

Woodson, R. E., Jr. 1954. The North American species of/ Asclepias/ L. Annals of the Missouri Botanical Garden 41: 1-211.


Carrie McLaughlin said...

Thank you for posting this for wide distribution, Sheryl. Lincoln provided this essay as the answer to one of the PowWow's guests who asked the question of him, and as you were at the conference, you will note that his response is much firmer than the one which Dara Satterfield gave when she spoke at the PowWow. For those who were not there, she simply advised that it be cut to within six inches of the ground in October (in Texas), and any green growing vegetation after that be immediately pinched off.

Also, please let your readers know that there is a genetic issue at stake in the study of monarchs. What their larva feed on will create genetic markers within the butterfly. It is very difficult to accurately determine those genetics if a wild monarch captured in Virginia, for example, has markers for a milkweed genotype from California because they fed in a garden seeded with milkweed from across the country. This happens most frequently with curassavica.

Unknown said...

This is such an old article and old argument. 2014 paper not relevant and not agreed to by many. I am always Leary of so called academic experts. Dr Glassberg of NABA has posted a rebuttal to this thinking. We need 1.5 billion more milkweed for the Monarch Butterfly do we really need a discussion about which milkweed is best. Plant all types of milkweed.

Sheryl Smith-Rodgers said...

I need to find Dr. Glassberg's thoughts and share them. My personal thoughts: We should plant milkweeds that are native to our own regions.

Paul Cherubini said...

Wild fall migrant monarchs in reproductive diapause ignore currassavica (and all other species of milkweed) except to drink the flower nectar. Brower is wrong that currassavica will cause all migrant monarchs in reproductive diapause to "come into reproductive condition" and it would be easy to document this with video footage.

Melody McMahon said...

Thanks for posting Lincoln's article Sheryl. I'm on the fence about tropical milkweed but I'm leaning toward pulling it up altogether. In my own garden as well as the WFSC gardens it needs too much water and looks awful much of the time. I just wish more nurseries sold the native milkweed to the public, especially the "big box stores" since that's where so many plants are sold to weekend gardeners. Education is the key!

Carrie McLaughlin said...

This is the TEXAS POLLINATOR POWWOW response to Mr. Grant's comment: "We addressed the important issue of using local ecotypes (genotypes) of all milkweeds (and other natives) at our pollinator conference last week through the research of Texas' USFWS State Botanist, Chris Best. I wish you could have been there to analyze what he said, William. I think you would then have agreed that Dr. Brower's statement cannot be casually swept away as irrelevant when his position directly and importantly addresses the issues of health and genetics for a diminishing North American species whose unique migration is endangered. We are fully aware that Dr. Glassberg disagrees, but we gave him and his supporters a great deal of encouragement and many invitations to attend the conference to publicly address the issue with Dr. Brower during the Q&A. They all declined to respond. Here is a very annotated CV for Dr. Brower that may help in understanding that he is not self-vaunted, but is in fact studiously learned in his life's work of almost sixty years with the monarch butterfly specifically: And we agree with you: plant all types of milkweed. But plant them in their proper place. Thanks for caring enough to write in, William."

Carrie McLaughlin said...

In response to Mr. Cherubini: You used a four-letter word in your comment, Paul. The word "all". Lincoln did not.

Rather than delve into the particulars for your readers, Sheryl, I'll give them this resource list to explore on their own, and from which they may draw their own conclusions about Dr. Brower's learned position, and Mr. Cherubini's opinion.

Of particular note is this one from the list:

Carrie McLaughlin said...

I would like to make a correction to my own comment, please. In my first statement on this post, I used the term "genetic marker" incorrectly. The correct term to use is "cardiac glycoside fingerprinting". Cardiac glycosides are contained in each plant which a monarch caterpillar feeds on, and those elements are used by the monarch to make them distasteful to their potential enemies, among other things. And glycosides can be fingerprinted such that you can tell geographically where the monarch has been feeding, and on what (assuming the monarch's food sources have not been interfered with as discussed previously).

Sheryl Smith-Rodgers said...

I recently received an anonymous comment for this post. Typically, I post anonymous comments if they're relevant and not off color. But for this post, I've decided to post only comments that are "signed." Reader, if you'd like to resubmit under your name, I'd be glad to post your thoughts.

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