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The Garden Scoop

Tropical milkweed could threaten monarchs

Tropical Milkweed with bees and a fly on it.

As interest in planting milkweed has grown among gardeners focused on supporting Monarch butterfly populations, many of us have been watching garden centers for milkweed plants.  This interest has sparked demand for easy-to-propagate milkweed species that can offer maximum garden beauty with a long season of bloom. 

Tropical Milkweed

Tropical milkweed (Asclepias curassavica) is one plant that fits the bill.  It is incredibly easy to propagate, compared to some of its native cousins that require cold stratification of seeds and can often have spotty germination.  Showy, two-toned flowers fill the plant from June to the first killing frost of fall, providing a long bloom time and spectacular flowering display. 

In our climate, tropical milkweed is considered an annual and must be replanted yearly, but it is a perennial plant in Zone 9 and further south.  In areas where it is perennial, it has escaped cultivation, invading forest edges, roadsides, field margins, and other open areas, displacing natives as it spreads.  Due to its rapid spread, there is some debate on its original home range since humans have interacted with the plant. Still, it is considered native to Mexico, the Caribbean, and Central and South America.

Every gardener that has become interested in supporting pollinators, such as the monarch, has heard that native plants support our native fauna better than non-natives and scientific evidence certainly backs up this claim.  Yet, the spectacular flowers on tropical milkweed and its long season of bloom are drawing in both gardeners and growers in pursuit of the best-performing milkweed in the landscape.  In recent years, this plant has become one of the most commercially available milkweed species nationwide. 

Tropical Milkweed Creates an “Ecological Trap”

As researchers assess the value of this non-native plant for our native monarch butterfly (Danaus plexippus), the evidence is mounting against it use in the US, with results indicating that this plant may be an “ecological trap” for monarchs.

It has the reputation as a “trap” due to its long season of blooms, the very reason many gardeners seek out this plant.  However, the longer bloom season has been shown to negatively impact monarch populations by disrupting reproduction and migration.  Tropical milkweed is in bloom during the late season, signaling to adult monarchs that it may be time to lay eggs and continue reproducing instead of migrating south.

In the late growing season, native milkweeds are nearing dormancy, and their foliage is of lower quality to monarch caterpillars.  In contrast, tropical milkweed does not have the mechanism to enter winter dormancy and remains a high-quality food source for caterpillars until killing frosts.  Research has shown monarch caterpillars feeding on higher quality milkweed late in the season, which reaches adulthood (or the butterfly lifestage), tend to remain reproductively active and do not pause reproduction, which is the natural precursor to migration.

This problem has been most prominently identified across the southern US, where the invasive tropical milkweed has established significant populations.  Researchers are finding that monarchs that feed on tropical milkweed (planted or invasive escapees) may delay or eliminate migration due to the year-round availability of high-quality milkweed leaf tissue.  This reduces overall monarch population numbers since these individuals cannot migrate naturally and wind up unsuccessful in the typical monarch lifecycle, which should include both spring and fall migration.

A Monarch Pathogen Perpetuated

This problem is compounding in the southern US due to a deadly protozoan parasite (Ophryocystis elektroscirrha) for monarchs.  Populations of tropical milkweed that do not die back to the ground (like native milkweeds) have been shown to perpetuate this monarch pathogen in their perennial plant parts.  In addition, monarchs are showing a feeding preference toward tropical milkweed over natives, which further attracts them to this tainted food source.

Tropical Milkweed Growing Worse with Climate Change

One 2018 research paper caught my eye as it attempted to predict the value of tropical milkweed in response to climate change and a warming annual temperature.  In this study, researchers at LSU compared swamp milkweed (Asclepias incarnata) to tropical milkweed.  To replicate a warming climate, some plants were fitted with open-top chambers that raise the temperature around the plant throughout the growing season. 

The researchers then measure the levels of cardenolides in all the milkweeds in the study. Recall that cardenolides are toxic compounds that milkweeds produce as a defense mechanism.  Monarchs have evolved to tolerate this compound as caterpillars and even incorporate it into their biomass as protection from predators. 

Past research has indicated that female monarchs select tropical milkweed over A. incarnata for egg-laying, indicating that females prefer milkweed plants with higher cardenolide concentrations.  However, females do not appear sensitive to the critical level of cardenolide concentration, which other work revealed to be around 3mg/g. 

In the LSU study, increased temperature exposure resulted in cardenolide concentrations around 4mg/g, and survival of monarchs on tropical milkweed was found to be five times lower.  After analyzing cardenolide levels in the lab, LSU researchers found that tropical milkweed exposed to increased temperatures produced up to 13 times more cardenolides than milkweeds not exposed to the higher temperatures. 

As our annual temperature increase with climate change, tropical milkweed is predicted to exceed the safe limit of cardenolides for monarch caterpillars.  Since female monarch butterflies are naturally attracted to milkweeds with higher cardenolide levels, we assume they will prefer tropical milkweed in the future, selecting it over native milkweeds.  This leads to yet another “trap” monarchs may experience with tropical milkweed since caterpillars may be exposed to toxic levels of cardenolides.

Should we plant tropical milkweed in Illinois?

In places like Illinois, tropical milkweed dies back in winter. So, adding this plant to our garden spaces does not risk creating a perpetual source of pathogens or overwintering, nonmigratory monarchs.

However, this plant is still not recommended since its late-season presence can disrupt migration behavior. Research in other states has shown that monarchs prefer tropical milkweed over native speices since it has a higher concentration of cardenolides, which suggests that monarchs in Illinois may also choose tropical milkweed over native species. It is unclear how much exposure to tropical milkweed is needed to disrupt migration cycles, but I don’t believe it is worth the risk in Illinois.

Since this plant is such a threat in southern states, it really needs to be phased out of garden centers nationwide. While there is likely less threat in Illinios since our climate cannot support perennial populations of the plants, any purchase we make here supports demand for this plant and supports its future in retail outlets.

If you have this plant in your garden now, consider pulling it out sometime in August so it does not present a high-quality food source later in the growing season. It’s difficult to remove a plant in full, beautiful bloom, but since it does pose a serious risk to monarchs, it is one plant that has to go!


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