What do nutria, tiger mussels, Chinese tallow, water hyacinth, kudzu and the apple snail all have in common? They are invasive species that have had a seriously negative impact on our native habitat.
Well, it’s time to add another to the list: tropical milkweed (Asclepias curassavica). It’s wreaking havoc on the globe’s already threatened population of monarch butterflies.
You might assume that, like forest-killing kudzu or bayou-choking water hyacinth, tropical milkweed would be the focus of serious eradication efforts. But no; often mislabeled as “butterfly weed,” which is a similar-looking native milkweed (Asclepias tuberosa), tropical milkweed is for sale in local nurseries in both red and yellow varieties. Adding to its spread, tropical milkweed is gaining popularity among landscapers of public green space. I have counted no fewer than 300 of these plants in Crescent Park, along the Bywater riverfront.
Most of us know what a monarch butterfly looks like. Its beauty is matched by an amazing lifecycle in which the Gulf South plays an important role. In late summer and early fall millions of monarchs, some from as far away as Canada, swarm through the region en route to Mexico. While on their way north in the spring, monarchs live from two to six weeks, but on the way south in the fall a “super generation” of monarchs live for up to eight months and can travel up to 3,000 miles to their overwintering ground in central Mexico. During this time they don’t normally reproduce, waiting until spring for fresh milkweed to sprout when winter is over.
Milkweed is the only species of plant that monarchs lay their eggs on and that their caterpillars eat. Which means the butterfly population has been devastated in the past 20 years by agricultural practices throughout the Midwest that have destroyed the native milkweed that is their breeding habitat. These practices include mowing patterns and the use of pesticides like Roundup. Add to that the devastation of the mountain forests in Mexico where the monarchs cluster and you’re looking at an icon of insect beauty, as well as a pollinator, in danger of extinction. In the past 20 years, monarch populations are estimated to have declined by 80 percent.
To the rescue … a disastrous misstep.
About five years ago there was a push to plant milkweed to help save the monarchs, and somehow tropical milkweed was introduced as the bright savior, likely because it’s so easy to grow and propagate. It’s now found in small nurseries and big-box stores across the country, and its use is encouraged on many save-the-monarch websites.
It’s one thing to plant tropical milkweed in the north where it dies off in the fall like most other plants. But it doesn’t die off here in our semi-tropical climate unless we have a hard freeze. The burgeoning presence of tropical milkweed tricks migrating monarchs into breeding throughout the Gulf South, rather than keeping their eggs and flying on to winter in Mexico as they have for thousands of years. Then comes the late hard freeze in our region and the now lingering, breeding monarchs are killed along with the milkweed. Their caterpillars may survive the cold, but without the milkweed, they soon starve to death.
An added problem is potentially an even greater threat to monarch survival. Tropical milkweed is, for reasons unknown, a major carrier of the spore-borne virus OE (Ophryocystis elektroscirrha). It causes horrible deformities in both the monarch’s chrysalis and butterfly stages, ending in death. The virus appears to be more prevalent on tropical milkweed because its longer growing season allows a greater number of butterflies to visit the plant.
OE is carried from plant to plant on the bodies of butterflies and other insects much like pollen, and some scientists believe it’s devastating the monarch population as much as the combined effect of 20 years of Roundup use and loss of habitat to agriculture.
Monarchs that actually make it to Mexico can spread the virus throughout the wider population as they huddle together in what has become, for better or for worse, an annual spectacle that attracts tourists from around the world. (It’s believed unregulated tourism is having a negative impact on habitat as well.)
I was surprised to see a deformed monarch this summer near the US/Canada border. That’s the end the line, literally. Deformed, they can’t mate or fly, and their presence that far north means the disease is spreading.
Another more common threat to monarchs are parasites, and because of the greater density of wintertime egg laying on tropical milkweed, these parasites can proliferate much more easily. The tachinid fly and the tiny trichogramma wasp both lay their eggs on monarch caterpillars or on their eggs, causing the monarch to die during or just ahead of the pupa (chrysalis) phase.
So, what can you do? A lot, actually:
No. 1: Don’t plant tropical milkweed; plant only native species.
No. 2: Learn to identify and protect native milkweed and ask commercial growers and dealers to eliminate the tropical variety from their inventories and sell only indigenous milkweed, if any.
No. 3: Plant fall flowers — not Asclepias curassavica! — that will provide nectar to migrating butterflies so they can complete their journey south.
If you already have tropical milkweed or discover it somewhere near you, you can do any or all of the following:
Cut it back several times during the summer. This will help lower the incidence of OE by keeping the new growth fresh. Then cut it back completely from early September through February and keep it cut back until early spring.
If it’s in a pot you can move it inside at the end of August, out of reach of migrating monarchs. If it’s in your neighbor’s yard, knock on the door, let them know, and ask permission to uproot it.
If you discover tropical milkweed in October or later, very likely there will be monarch eggs and/or caterpillars on it. So leave it alone! If you cut back the milkweed, the caterpillars or eggs will starve.
Right now, with winter bringing the first real frosts, is the time to get serious about eradicating tropical milkweed in the region. With every good intention, we unwittingly introduced this threat to the monarch butterfly’s survival. We have a chance to reverse the damage if we act quickly and intelligently to root out the plant. Uproot now or at the end of August, and spread the word!
Shawn Hall is a multidisciplinary artist, painter, and a budding citizen scientist/naturalist. A Michigan native, she has lived in New Orleans since 1997.