The Issue
Tropical milkweed (Asclepias curassavica) has been in the news recently. It can be a host plant for Ophryocystis elektroscirra (OE), a parasite of monarch butterflies (Danus plexippus). Boiled down, milkweed is a host plant for monarchs which carry the OE parasite. The butterfly deposits OE when they land on the plant to lay eggs. Newly hatched monarchs eat the plant and spread the parasite. The OE may persist on the plant as it grows through winter and spread to other monarchs. This parasite affects migratory patterns and though rarely lethal can hinder pupa emergence and shorten lifespan. In recent decades, monarch populations have dwindled making OE is an unneeded stress. There are several reasons for the decline, most notably loss of 862 million milkweeds in the Midwest between 1999 and 2014 (Pleasants, et al 2017).
Tropical milkweed and butterfly migration in Florida
Monarch butterfly migration is unique in the insect world. Traveling between sites in the northern U.S. and Mexico, it rivals many birds in terms of distance. Day length, temperature, and resource availability trigger their journey. The later of these is where the tropical milkweed comes into play. A fast grower and well suited for Florida’s environment they are readily available and planted in many home landscapes. It is an excellent food source and habitat for monarchs and other pollinators.
In South and parts of Central Florida, the milkweed foliage remains year-round encouraging monarchs to cease their migration. Butterflies then gather spreading OE which uses the plant to survive infecting the next generation of insect. In North Florida, consistent winter freezes takes this milkweed to the ground. With no host, and nowhere to overwinter, the OE dies back. You may then wonder if removing this plant will prevent the parasite. OE infects around 5.5% of the eastern monarch population (Majewska et al, 2022) and can be deadly. Habitat loss from culling this plant has the potential to be deadlier still as seen in the Midwest. So what is the answer?
Planting native milkweeds is absolutely the best mitigation strategy. They follow seasonal cycles encouraging the butterfly to continue its journey with no input from the homeowner. Unfortunately, they are not readily available in most garden centers. Tropical milkweed is a non-native alternative to fill this void. While great in this capacity, they require extra management. You’ll need to cut this plant within four inches of the ground once flowering has ceased in autumn (generally around Thanksgiving). Doing so mimics the seasonality of natives with much the same result. There are also concerns of a potential invasive status particularly as our climate changes. As of this writing, University of Florida does not recognize this plant as invasive though caution is encouraged in the south portions of the state.
Final Thought
Do not let all the negativity get to you. There are good things happening with monarchs. Over the 2021-22 winter nesting grounds saw a 35% increase according to the World Wildlife Foundation. It’s a small bump, but it’s a start. As monarchs start their road to recovery it will be important to provide food and habitat. Utilizing milkweed in your gardens is crucial in that effort. Tropical milkweed is a valuable tool, but requires proper management. For more information on monarchs, see this Ask IFAS document, or contact your local extension agent for additional information on this and any topic regarding your gardens and more.
Resources:
Majewska, Ania A. et al. “Parasite Dynamics in North American Monarchs Predicted by Host Density and Seasonal Migratory Culling.” The Journal of animal ecology 91.4 (2022): 780–793. Web.
Mueller, Elisha K, and Kristen A Baum. “Monarch–parasite Interactions in Managed and Roadside Prairies.” Journal of insect conservation 18.5 (2014): 847–853. Web.
Pleasants, John, Simon R. Leather, and Alan Stewart. “Milkweed Restoration in the Midwest for Monarch Butterfly Recovery: Estimates of Milkweeds Lost, Milkweeds Remaining and Milkweeds That Must Be Added to Increase the Monarch Population.” Insect conservation and diversity 10.1 (2017): 42–53. Web.
Reppert, Steven M, and Jacobus C de Roode. “Demystifying Monarch Butterfly Migration.” Current biology 28.17 (2018): R1009–R1022. Web.
World Wildlife Foundation
https://monarchconservation.org/wp-content/uploads/2022/05/Monarchs-Winter-Population-21-22.pdf
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