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.