The relative virtues and problems associated with tropical milkweed, Asclepias curassavica, continue to be a hot topic within the monarch conservation community, but the disparity between the two is becoming more and more clear. Scientific research suggests that its problems, namely its link to the spread of the Ophryocystis elektroscirrha (OE) disease, far out-weigh its virtues. In fact, it’s those very virtues—availability, adaptability, and long bloom season—that multiply its negative effects relative to the health and sustainability of the monarch butterfly species.
What is OE?
OE is a protozoan parasite that infects butterflies that host on milkweed. Its life cycle starts as a microscopic spore that breaks open when ingested by a caterpillar. Within the caterpillar, it grows and multiplies. Because a parasite depends on its host for its own life, OE rarely kills the caterpillar.
But the disease affects the development of the adult butterfly while pupating, and adults emerge weak and often with crippled wings. While many monarchs may carry OE as spores attached to its wings and thorax, the size of the spore-carrying population and the heavy level of spores within that population in the Gulf Coast region—especially Texas and Florida—is cause for alarm. Visit Project Monarch Health for more about OE.
Recent studies corroborate earlier studies and tighten the link between tropical milkweed and an increase of OE. Gardening to help conserve monarchs requires an understanding of the risks associated with tropical milkweed as well as the steps to take to minimize its ill effects.
The introduction of tropical milkweed in the U.S.
Monarch enthusiasts with the best intentions were thrilled when local nurseries began to offer tropical milkweed for sale and embraced the Mexican native with gusto. It didn’t take long to discover that aside from being very easy to grow, monarch butterflies love this variety of milkweed. It seemed that a solution was in hand to help restore milkweed habitat for the Eastern migratory monarch population. As a result, tropical milkweed has been well established in parts the southern states—especially southeast Texas and southern Florida.
Then research began to emerge that showed an increase in monarch disease caused by OE was linked to tropical milkweed grown in the southern states.
What the research shows is particularly troubling for the monarch migration that passes through Texas gardens to feed and breed.
The effects of tropical milkweed
Research by Karen Oberhauser, Dara Satterfield, and others has and continues to demonstrate that OE in monarchs increases where tropical milkweed flourishes. (See links to studies at the end of this blog.)
What’s been determined is that the proliferation of tropical milkweed (in the southeastern parts of Texas and south Florida in particular), coupled with its near year-round foliage and flower production does two things:
It interferes with the monarch’s migratory cycle. Tropical milkweed encourages them to linger in the southern states and continue breeding and laying eggs, “trapping” them here where they cannot survive temperatures that drop toward the freezing mark. Possibly more important is the effect of milder winters. Given a non-stop supply of milkweed, interference with normal migratory behavior produces populations of monarchs that overwinter in Texas and Florida instead of completing their migration to the oyamel fir tree forests of central Mexico.
Monarchs who stay in the southern states for the winter are five to nine times more likely to be infected with OE than migrating butterflies.
It significantly increases the rate monarchs are infected by the debilitating OE protozoan pathogen. If mild winters don’t produce a freeze, infected milkweed continues to thrive, not dying back like native milkweed species. This means infected plants persist. Infected plants in Texas are especially harmful because they sit in the gateway for the spring and fall monarch migrations.
Migrant butterflies at sites with overwintering residents were 13 times more likely to have infections compared to migrant populations that don’t come in contact with residents.
Adult monarchs migrating from Mexico in the spring that visit infected plants pick up hundreds of OE spores and carry them to other plants—increasing the number of infected plants and as a result butterflies, exponentially.
What to do?
If there’s any good news in this it could be that originally, most of the tropical milkweed planted was done so in gardens. By definition, gardens are tended. Gardeners should consider taking one of two actions.
Replace tropical milkweed with native species. While native varieties are more challenging to start, the effort would help minimize the spread of OE. Try these native species:
- Asclepias incarnate, Swamp milkweed
- Asclepias perennis, Aquatic milkweed
- Asclepias tuberosa, Butterflyweed
- Asclepias verticillata, Whorled Milkweed
- Asclepias viridi, Green milkweed
Or, be diligent about cutting it back every winter. Cut tropical milkweed plants to within four to six inches of the ground each November.
If you have tropical milkweed in your garden and didn’t cut it back in November, do it now.
Milkweed for habitats
Milkweed used for non-gardening purposes poses a more clear guideline. Dara Satterfield recommends, “that habitat restoration for monarchs focus on native species of milkweed, which are synchronized with the monarch’s natural migratory cycle and do not enable the year-round breeding that can lead to high parasitism rates.”
The spring migration approaches
Tracking the spring monarch migration starts on February 14. Visit Journey North to learn how you can enter your own monarch sightings and track the migration real time.
Delve in and learn more about tropical milkweed and its effect on the health of monarchs with these recent studies:
Patterns of parasitism in monarch butterflies during the breeding season in eastern North America, Ecological Entomology, 2018
Monarch butterfly migration and parasite transmission in eastern North America, Ecological Society of America, 2011
Loss of migratory behaviour increases infection risk for a butterfly host, The Royal Society Publishing, 2015
Learn more about native milkweed species at these resources:
PDF of Identification of Milkweed in Texas, by Texas Parks & Wildlife