Tropical Milkweed is Harmful to Monarchs & Florida Ecosystems
By Lilly Anderson-Messec
Unfortunately, tropical milkweed has been an increasingly invasive species in Central and South Florida for many years, and has begun spreading in North Florida as well. It’s fast growth and prolific re-seeding have resulted in large monocultures of tropical milkweed in natural areas. This unchecked growth replaces native plants and disrupts the native ecosystems that both wildlife and humans rely on. The invasive quality of this plant is is just one of the reasons we recommend removing tropical milkweed in your yard. Unlike our native milkweed species which naturally senesce in the fall, the lush green foliage of tropical milkweed will stay up all winter if not killed back by frost, which has become a problem for the already imperiled monarch.
A protozoan parasite that evolved with monarch butterflies, Ophryocystis elektroscirrha (OE) lives on infected monarchs and is deposited on the plants they land on; especially when the butterflies lay eggs on the plants. The resulting caterpillars hatch and ingest the OE as they eat the plant, and the parasite is able to replicate inside them. Those caterpillars will grow into butterflies infected with an increased load of OE that they will shed on other milkweed plants they land on, continuing the cycle.
Many species across the animal kingdom have evolved with their own particular parasites – humans included. These parasites are often not too harmful to their host since their own survival is dependent upon the survival of their host. However, if the delicate balance between host and parasite is suddenly effected by a change in their environment, one of the pair may be given an advantage. If the parasite is allowed to accumulate too much, it can kill their host. Monarchs evolved with OE and are naturally able to prosper while still carrying a small amount of the parasite, but high OE levels in adult monarchs can cause them to fail to emerge from their pupal stage because they are too weak and unable to fully expand their wings. Monarchs with slightly high OE loads can appear normal, though they are usually somewhat smaller in size. Despite this survival, they don’t live as long, cannot fly as well, and are therefore unable to migrate successfully.
Our native milkweeds naturally senesce in the fall and stay leafless and dormant through the winter, which effectively cleans the plant of the seasonal OE parasite load. When the leaves die back, the parasite dies along with them so that when butterflies return each spring and summer, they feed on fresh, parasite-free foliage. In contrast, tropical milkweed remains evergreen throughout the winter, allowing OE levels to accumulate on the plant. The following generations of monarch caterpillars that feed on those plants are exposed to dangerous levels of OE.
As our winters have become increasingly warmer in North Florida, the invasive potential of tropical milkweed is growing. Warmer winters also mean tropical milkweed is less likely to be killed by frost and more likely to accumulate excessive OE on its leaves. Tropical milkweed can also interfere with monarch migration and reproduction. In northern areas it grows later into the season than native species do, and some studies have shown that just the presence of tropical milkweed may confuse monarchs into breeding at a time when they should be migrating. There is evidence that suggests the chemical composition of tropical milkweed may trigger this disruption of the innate migration cycle of the monarchs that interact with it. This creates a trap for monarchs, as they are fooled into thinking that they have arrived in the safe wintering grounds of Mexico, when they are not - and the inevitable winter freeze kills them.
With the mounting evidence of the detrimental effects of tropical milkweed, many organizations involved in monarch conservation, such as the Xerces Society and Monarch Joint Venture have begun recommending against planting non-native milkweed - even going so far as to recommend NO milkweed if native species are not available. Many native plant nurseries have heeded the call to stop selling non-native milkweed species. Unfortunately, many growers are still working to build adequate supplies of stock to meet the ever-increasing demand. Though supplies are limited of native species, we would recommend that it is better to be without milkweed than to buy or sell tropical milkweed. Likewise, if you have it in your yard, or notice it in natural areas - please consider removing it. While it may feed our monarchs in the short term, it is harmful to the species as a whole in the long term.
If you cannot find native milkweed species at your local garden center, request it! Be specific, and ask for native Florida eco-types by their scientific name (some species recommendations can be found below). You may have more success if you can find a nursery near you that specializes in native plants. You can find a list of native plant nurseries across the state, and even see what plants they carry at PlantRealFlorida.org - just click on ‘Retail Nurseries/Garden Centers’, select your county, and then you will see a map of nearby nurseries and can check their plant lists. It is best to call the nursery before you go to be sure of current availability, which may change daily.
Which Florida Native Milkweed Should I Choose?
Pink Swamp Milkweed, Asclepias incarnata
Photo by Lilly Anderson-Messec.
PINK SWAMP MILKWEED
WHITE SWAMP MILKWEED
BUYER BEWAREWhen purchasing any native plant at a nursery, you should always know what you are looking for and how to identify the plant before you buy - nurseries and growers make mistakes and you may end up buying and planting an invasive species. Be especially careful when purchasing Asclepias tuberosa at large box stores that have been known to erroneously sell Tropical Milkweed, Asclepias curassavica, labeled as our native Butterflyweed, Asclepias tuberosa. You can easily differentiate the two by looking at the leaves and stems (see comparison photos below). Also, true Asclepias tuberosa does not produce a noticeable amount of the sticky latex sap when a leaf is broken, while Asclepias curassavica will.
Few-flower Milkweed, Asclepias lanceolata is another native species that can often be confused with Tropical Milkweed. It is not commonly available in the horticulture trade, but if you think you have found Tropical Milkweed in a natural area, make sure to confirm it is not actually Asclepias lanceolata before you remove it! Asclepias lanceolata flowers look very similar to A. curassavica flowers, but the leaves of A. lanceolata are much longer and more narrow, and the plants are generally more lanky and tall - see photos below for comparison of these species.
- List of 46 conservation areas in Florida reported by the Institute for Regional Conservation to have naturalized populations of Asclepias curassavica; https://regionalconservation.org/ircs/database/plants/PlantPage.asp?TXCODE=Asclcura
- Reseach article: Climate change and an invasive, tropical milkweed: an ecological trap for monarch butterflies https://esajournals.onlinelibrary.wiley.com/doi/pdf/10.1002/ecy.219
- oss of migratory behaviour increases infection risk for a butterfly host https://royalsocietypublishing.org/doi/full/10.1098/rspb.2014.1734
- Xerces Society: Tropical Milkweed—a No-Grow https://xerces.org/blog/tropical-milkweed-a-no-grow
- Rearing Monarchs Responsibily https://monarchjointventure.org/images/uploads/documents/Monarch_Rearing_Instructions.pdf
- Share this fact sheet with local growers, farmers and nurseries to encourage them to grow native milkweed: https://monarchjointventure.org/images/uploads/documents/Grow_and_Sell_Milkweed_Fact_Sheet_Final.pdf
- Research article: Exposure to Non-Native Tropical Milkweed Promotes Reproductive Development in Migratory Monarch Butterflies, Ania A. Majewska and Sonia Altizer https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6724006/