The Tropical Milkweed

Where I grew up, the common milkweed, Asclepias syriaca, was part of my environment. I was used to big plants like this

and large pods like this

(My Platonic Ideal Milkweed is shown in a copyrighted image here, under Asclepias syriacea.)

Imagine my surprise when a visiting botanist pointed out the “blood milkweed” - a delicate red and orange flower reminiscent of Latana (and more on that similarity another time) - in my own back yard.

With a botanist at hand, there was no problem identifying this plant: Asclepias curassavica, the tropical milkweed, of the family Asclepiadaceae. I will work up the characteristics of the plant that make it a member of the Asclepiadaceae shortly, but first a mention of the most famous relationship milkweed has with an insect - the Monarch butterfly.

I’ve drawn much of my information about this relationship from the Monarch Butterfly Fact Page. Female Monarchs lay their eggs only on milkweed plants, attaching about 400 “clear green oval eggs” to the underside of a leaf. The caterpillars

emerge a few days later. In the United States, east of the Rocky Mountains, these caterpillars mature into butterflies so quickly that several generations will be produced in early to mid-summer. The generation that emerges in late August, though, will live eight or nine months and will migrate to Mexico. At the end of their life, the Monarchs will reach sexual maturity and will migrate back to the US, mating all along the migratory route.

No other butterfly in the world migrates like this. They often migrate to the same exact trees to which their ancestors (several generations ago) migrated last year.

Other than their spectacular migration behavior, Monarchs are known for being repulsive to predators. I remember a famous picture in a college textbook of a Blue Jay throwing up after having eaten a Monarch butterfly. The bright colors of the Monarch will help birds remember not to eat that butterfly again.

Now I’d like to look at the Milkweed Family, Asclepiaceae. The family is named for the Asclepias genus, which in turn is named for Asklepios, the god of healing in Greek mythology. Milkweed roots have been used medicinally in the tropics for pain relief and for the treatment of scrofula.

The common name for the family, Milkweed, refers to the white latex mentioned earlier. This production in combination with simple opposite leaves with no teeth on their edges are good markers for the family.

The flowers, though, are unusual enough to make many members of the family recognizable on sight. As Elpel describes it, there are 5 separate sepals, 5 united petals, and a “corona” (like a crown) that looks like an extra set of petals. The corona consists of 5 hood-like forms facing toward the center of the flower. Inside the corona are 5 stamens fused to the ovary. In A. curassavica, the crowns are yellow and the petals are a red-orange color.

These flowers also have some features, which I’ll not describe, in common with orchids, although the two families are not related. (Note the ants in the image above. They are robbing nectar!) You can find excellent copyrighted illustrations that label of all these parts at Digital Flowers.

The fruit is that famous milkweed pod loved by children because silky threads are attached to the seeds inside. In this image, you can see a pod in front of the inflorescence, and a developing pod behind the inflorescence:

So my checklist for this family is: 1) simple opposite leaves with no teeth on their edges, 2) milky latex in stems, 3) complex flowers with crowns.

My final question is whether Monarchs like all flowers of the Asclepias genus equally or whether, the “common milkweed,” A. syriaca, is the Monarch’s favorite. Naturally, having grown up with the common milkweed, I assumed that was the best for the butterfly.

Luckily, I’m not the first to have had that thought. Some students from Willow Creek Middle School and Century High School in Rochester, MN, had a similar question. They went beyond my simple question, though. They raised 100 Monarch butterflies from eggs, 20 on each of 5 different species of Asclepias. They found that larvae fed on the tropical milkweed (A. curassavica) had the highest survival rate and the shortest time to pupation. The common milkweed was in the middle. So much for my provincialism!

What remains is for me to spot a Monarch on my plant, or even a Monarch migrating. It won’t be one that has migrated from the United States. Central American Monarchs migrate between the highlands and the lowlands only, and they migrate according to wet and dry seasons.

Ready when you are, Monarch!

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