Saturday, January 17, 2015

Tropical Milkweed - Friend or Foe?

Tropical Milkweed, Asclepias curassavica
This week, several news articles appeared based on a recent study questioning the wisdom of planting Tropical Milkweed, Asclepias curassavica. In many cases the headlines were sensational and pointed fingers at home gardeners as villains. I'd like to take a few moments to sift through some of the misunderstandings and urge calm garden practices. The study is very well done but only focuses in areas home to Eastern monarchs where native milkweeds die back each winter. Even so, it is important to review their findings and see how it applies to us in the Southwest.

Monarch larva in Zitacuaro
Scientists have long voiced concern about the increasing number of monarch butterflies overwintering in the southern United States, particularly in Texas and the Gulf Coast. We all know monarch butterflies are dependent on milkweed (Asclepias) as their only host plant. The recent plummet in the monarch population has encouraged more people than ever to grow milkweed to help the monarch population increase. But native milkweeds aren't always easy to find. More and more stores are now only selling Tropical Milkweed, also known as Bloodflower (and other common names), Asclepias curassavica. This milkweed is native to Mexico and is also found near the overwintering sites. Last winter in late February we found monarch larvae at the hotel where we were staying in Zitacuaro in Mexico shortly after finding monarchs mating at the nearby overwintering sites.

Tropical milkweed is easy to grow from seed and so it has become popular with home gardeners. Monarchs love it, in fact many people feel it is monarch candy - monarchs can't resist it. In most parts of the country, native milkweed dies back during the winter months. Tropical milkweed is frost tender and will die back in areas with a hard freeze, so it is grown as an annual in the northern states. But in some areas in the south,  A. curassavica can thrive during the winter months. Breeding monarchs will find the milkweed and lay their eggs profusely, often devouring all the leaves, stripping branches bare.

So what's the problem? Enter in the rising concern about Ophryocystis elektroscirrha (O.e.). Studies have shown that high levels of O.e. are lethal to monarch butterflies. O.e. is higher in year round monarch populations. Mating with an infected monarch spreads O.e. Spores from an infected monarch can be present on milkweed leaves and then ingested by monarch larvae increasing the incidence of O.e. in the population as well. It is a rising concern and exactly why this study was done.

I've received many emails and FB posts on the Southwest Monarch Study Facebook page with people in a panic, urging everyone to rip out A. curassavica in their gardens. I'm seeing good-hearted and passionate people creating a great divide and finger pointing. But in this time of low monarch populations, I'd like to urge everyone to take some time to step back and think things through a bit. First, everyone who is growing milkweed, no matter the species, should be commended for their efforts to grow the monarch butterfly's only host plant. Right now increasing habitats is number one and you are part of the solution. Second, we need to urge plant nurseries to grow more native milkweeds. There is a serious lack of options in many parts of the United States.

Desert Milkweed, Asclepias subulata
Native milkweed seeds can be difficult to grow, more tutorials (both written and in videos) need to be available for optimal success. We recently posted one on our Southwest Monarch Study Facebook page featuring an easy way to start Desert Milkweed, Asclepias subulata, that does not require cold-stratification. In order to do this, more native milkweed seeds need to be available. Luckily, the Xerces Society recently stepped in to help with their Milkweed Seed Finder. Or, you can order milkweed plugs from the successful Monarch Watch Milkweed Market.The Southwest Monarch Study will offer free Desert Milkweed seeds soon on our FB page.

Why am I urging calm here in the Southwest? For one simple reason. So far, here in Arizona, O.e. levels are very low, an average of 4.3% over four years. The infected monarchs were all in Southeast Arizona breeding grounds, not in the small number of overwintering monarch populations. In the West, O.e. levels are predicted to be about 30%. We recently included this information in part of a larger paper for publication that is currently in peer review. In the chart below, only category 3, samples with more than 100 spores are considered highly infected by Monarch Health.
Results of O.e. testing by Monarch Health of monarchs in Arizona

 There are three important questions that need answers:
  • Is it Tropical Milkweed, Asclepias curassavica, that is a concern, or is it any milkweed? In the Southwestern low deserts we have four native evergreen milkweeds: Desert Milkweed (A. subulata), Pineleaf Milkweed (A. linaria), Arizona Milkweed (A. angustifolia), and Asclepias albicans. It isn't uncommon to see a few late monarch stragglers fly through and lay eggs on these milkweeds in December and January. We offer a unique opportunity here in the Southwest to learn more about monarchs during the winter. We are currently monitoring two sites and hope to share this information in the future.
    Pineleaf Milkweed, Asclepias linaria
  • What is the effect of temperature on monarch breeding in the fall? 2014 was the warmest on record. Is it the presence of milkweed or is it warmer temperatures that trigger breeding in late eclosing monarchs in November and December?
  • Where is Tropical Milkweed naturalizing? So far in Arizona, Asclepias curassavica is only an irrigated garden specialty. We have no evidence of naturalizing in our extreme dry climate and temperatures. Continued monitoring is crucial as this could change over time. Furthermore, native evergreen milkweeds far outnumber the small amount of Tropical Milkweed available.
Desert Milkweed, Asclepias subulata
What should you do?
  1. Plant milkweed! If you only have Tropical Milkweed, add natives this Spring. Several are now available in Tucson, Phoenix and Yuma. Native Desert Milkweed, Asclepias subulata, is a monarch favorite. It is a common landscaping plant as well.
  2. Monitor monarchs using your milkweed as a host plant. If you have larvae, test freshly eclosed adult monarchs for O.e.
  3. Encourage your local plant nursery to carry NATIVE milkweeds! We are so lucky to have so many native species that thrive in our hot, dry climate in full sun.
  4. Grow native milkweed and share them with friends, relatives, coworkers, etc. Encourage your workplace, church, nature center and city parks to include milkweed in their plant selections. If they are adding plants for a more pleasing landscape, why not native milkweeds?
  5. If you grow Tropical Milkweed be sure to cut it back periodically.
I hope this helps. I want it to be clear I am not in disagreement with this study, in fact I'm very supportive and glad this is finally published. So let's use this information to benefit the monarchs and learn more about their life cycle here in the Southwest.


  1. Can you please clarify what you posted above? Are you speaking to people who live in the Southwest region about not growing tropical milkweeds, or are you also speaking to people who live in the Eastern and Mid-Atlantic regions about not growing tropical milkweed. I'm in Mid-Atlantic area and am a little confused but am dedicated to helping the Monarchs.....

    1. Here are some questions to think about where you live: What time of year do native milkweeds naturally grow in your area? Is A. curassavica (Ac) the only milkweed or are others common as well? Do you have monarchs that spend the winter? Do you have freezes that kill back all milkweed, including Ac? If you live in a location where you have freezes that kill back Ac, you shouldn't have any concern at all. Ac doesn't cause O.e. - it is infected monarchs that can drop spores on the plant that larvae consume. It is always a good idea to plant several species of milkweed in case weather or disease affects one kind eliminating it. But do plant milkweed and fall nectar to support the migration as well! Hope this helps.

  2. Thank you for a most thoughtful and reasoned post.
    Thought you might find this interesting Dr.Fred A. Urquhart's book The Monarch Butterfly: International Traveler page 98, he states that in 1951 he traveled to the peninsula of Florida and found monarchs there in winter, he also found them in California and Mexico and concluded "that not all monarch butterflies migrated."

    So despite dire warnings it seems that monarchs have lived as a non migrating species in Florida for some 64 years. And OE has not eliminated them.

    1. Thank you for sharing. There is a long history between monarchs, A. curassavica and O.e. that we all likely need to learn more about. This study can help us all learn more as a result and hopefully at the same time help us all learn about the native milkweeds where we live as well.

  3. Regardless of the threat that A. curassavica and O.e. pose on monarchs, we should care about the conservation of milkweed species as much as we care about conserving butterflies. Several native species are locally threatened or endangered. Planting an introduced species doesn't help them at all, does it? Granted that it is not for the ordinary gardener to find seeds and successfully grow the native species that are most endangered, but at the very least we should be aware of this matters and see what we can do.

  4. That is exactly why we strongly advocate for native milkweeds in all restoration. Until more native milkweed seeds are available, however, it could be a disservice for everyone to panic and yank out any A. curassavica they already have in their home gardens. It will be a process and should be monitored as I've suggested.

    We recently posted planting guides for Arizona on the Southwest Monarch Study website as well as a link re A. curassavica on our milkweed page. As I've pointed out earlier, we already have evergreen native milkweeds in Arizona.

    Thanks for your comment and insights.

  5. Many including you have suggested there is a significant over wintering of monarchs in the southern states. If you check reference 32 of the georga OE study published by the British society you will find that the reference to overwintering is based on a 9 year citizen science study that found in 9 gulf states an average of 27 monarchs per year. This hardly seem enough to conclude that a significant over wintering population exists in the south.

    1. This is the study referred to: Howard, E., H. Aschen, A. K. Davis. 2010. Citizen science observations of monarch butterfly overwintering in the southern United States. Psyche doi;10.1155/2010/689301. I suggest you email Journey North for more information and the authors. In general I can say that reported sightings are usually just that - sightings that people take the time to report. They are an indicator and usually there are many more. In the Phoenix area there are only a few reported to JN each winter yet we always have far more posted or emailed in. We give these numbers to JN seasonally. So the numbers in the study are not exact numbers but an indicator of activity. From earlier Dplex posts and sightings around the greater Phoenix area in some years there are hundreds. Hard freezes are the limiting factor.

  6. Last winter I had A. subulata in my garden and had no monarch laying. This fall, I added a few A. tuberosa and A. curassavica. I had an unbelievable number of butterflies eclose. It is now the end of March, the plants are recovering from being literally eaten to the ground, and the butterflies are gone. My plants may not freeze to the ground here in Palm Springs, but they are composed of completely new growth.

  7. Thanks for your note. Yes, more milkweeds usually increases the number of monarchs and larvae you will see. Congratulations on adding more to your garden. What milkweed did you see used the most? You'll likely notice variation from year to year depending on temperature.

    You raise a good point that monarch larvae will strip all the leaves off milkweed and this can have the same effect as cutting branches back in A. curassavica. It isn't a good idea to cut back branches of evergreens like A. subulata as it affects their growth.

    You are at an important doorway for both the spring and fall migration. Let me know if you are interested in tagging - and be sure to look for tags. There is a sizable overwintering site not too far from you in Rancho Mirage that uses our tags. When breeding the monarchs use A. subulata that is abundant there. If you are interested, email

    Thanks for commenting.