Asclepias curassavica – Tropical Milkweed

This particular milkweed has been the focal point of some recent controversy, so I thought I would put together as much research as I could find on this Monarch favorite.

Definition – Ophryocystis elektroscirrha (OE) is a protozoan parasite that infects Monarch and Queen butterflies.
 http://monarchparasites.uga.edu/whatisOE/

Karen Oberhauser indicates that in Southern and Costal States + California – that OE can be a problem as the plants don’t die and OE can build up. Some sites advise to cut it down to the ground at the end of the season if you are in one of the warmer climates.

Thanks to Andy S. with this article from SciencMag.org. It again talks about the Southern State problem with OE and what can be done.

Here’s a recent TedTalk which seems to indicate that Curassaivica is good for Monarchs and is in fact medicinal against OE.

I emailed Chip Taylor, a well known expert in this field, about this video and this is his response.

“I haven’t had time to view the TED but here is my take from what I know about the issues.

There is a twist to the story – or interpretation. Monarchs surviving on A.c. having acquired an infestation of spores – are able to mate and pass on the spores to offspring since spores would be transferred to the new eggs or surrounding leaves. Monarchs receiving similar doses of O.e. spores but which have fed on other milkweeds would likely die – ending transmission to the next generation.

If you follow this, O.e. is a self-limiting disease (but you will never hear the researchers talking about it in these terms). Mortality/viability is dose related. Too much and you are dead, a little bit and you live to carry to spores to the next gen. A.c. allows more to survive and hence favors spore transmission.

That’s an issue in the south where A.c. overwinters (almost exclusively in gardens so the scale of the problem is quite limited) where O.e. can build up on leaves. It is less likely to be an issue in the north. Cutting back A.c. in the south at least twice a yr is recommended. It wouldn’t hurt to do that in the north if expecting to produce more than one gen on A.c.”

Dr. Jeffrey Glassberg, the President of NABA, North American Butterfly Association, wrote an article in the Winter 2014 issue of American Butterflies. Here’s the PDF. He says, “There is little evidence to support the idea that planting Tropical Milkweed will weaken Monarch populations and NO evidence to support the idea that Tropical Milkweeds are trapping Monarchs and stopping them from migrating to Mexico.

Here is a AskNature Article with the same information as the video.

Cardenolides per Species – interesting that Curassaivica has a very high percentage.

• Asclepias curassavica – 1055
• Asclepias syriaca – 50
• Asclepias incarnata – 14
• Asclepias tuberosa – 3
• Asclepias verticillata – 1

San Antonio note – I got this email from a woman who lives in San Antonio. She said, “Monarchs are staying at the A. currasavica milkweed patch on the San Antonio Riverwalk all winter.” If you also look at the map below, you will see a lot of Monarchs do seem to stay in the southern states over the winter. Curassavica does seem to be stopping these Monarchs from migrating.

 

My summation –  it always makes sense to be aware of the OE problem and if in the South or California to take measures to limit use of Asclepias curassavica or at least cut it back a couple times a year. While only 8% of migrating Monarchs have OE, 85 % of the non-migrating Florida population have it. 

In Missouri – since tropical milkweed dies with the frost, spore transmission is not a problem.  What I have observed over the last 18 years is that even though I have a large tropical milkweed patch in October, as soon as we get a cool spell and the winds are moving in a southerly direction, the Monarchs take off on their migration.