Milkweed Toxin Aids in the Survival of Monarch Butterflies

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A recent article in the Journal of Parasitology explores the relationship between parasites and their hosts. The study examines various stages of the parasite life cycle and how monarch butterflies utilize toxins in milkweed plants to ward off parasites.

The Journal of Parsitology Cover

The Journal of Parsitology: Volume 103, Issue 3

we were surprised to discover that medicinal milkweeds could change the morphology of monarch parasites

The Journal of Parasitology – Parasites are some of the most common and diverse organisms in the world. Studying the relationship parasites have with their hosts gives insight into their effects on wildlife, domestic animals, agriculture and humans. By observing parasites through their life stages, researchers are able to identify how they use their morphological changes to their advantage. This understanding may assist in reducing the effects parasites have on their hosts.

Researchers from Emory University in Atlanta, Georgia, recently published a study in The Journal of Parasitology in which they took an innovative approach to studying host–parasite interactions. Realizing that many species will use a medicinal approach to combat parasite infiltration, they observed how the monarch butterfly utilizes toxins in milkweed plants as a form of resistance to parasites. By focusing on the spore stage of the parasite life cycle, the researchers were able to track the effects of the milkweed on the morphology of the parasite.

Forty-one monarch butterflies and three different types of milkweed plants were used in the experiment. The researchers measured the concentration of cardenolide (a type of steroid) in each plant type. During the butterfly larvae stage, parasite spores were introduced into the butterfly’s development. The goal was to study the effect that the host’s diet had on the morphology of the parasite.

The researchers found that higher concentrations of cardenolide reduced parasite spore sizes, which left them more unprotected against their host’s defences. For the host, this increased its overall fitness by increasing its tolerance to the parasite. The researchers state, “While we’d known for some time that milkweed toxins could help monarch butterflies combat their parasites, we were surprised to discover that medicinal milkweeds could change the morphology of monarch parasites. As far as we know, this mechanism hasn’t been observed previously, and it provides a new avenue for research on the effects of self-medication on parasite fitness.”

Studying how the host’s diet affects the morphology of the parasite shows that this type of medicinal approach can greatly increase the tolerance of the host and help combat the parasite. The researchers suggest further study to compare the effects of reduced- and normal-sized parasites on the monarch butterfly–parasite interaction.

Full text of the article, “Host Diet Affects the Morphology of a Butterfly Parasite,” Journal of Parasitology, Vol. 103, No. 3, 2017, is available at


About the Journal of Parasitology
The Journal of Parasitology is the official journal of the American Society of Parasitologists (ASP). It is a medium for the publication of new original research, primarily on parasitic animals, and official business of the ASP. The journal is intended for all with interests in basic or applied aspects of general, veterinary, and medical parasitology and epidemiology. For more about the journal or the society, see

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Caitlyn Ziegler
Allen Press
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