The Milkweed’s Self-Defense
Over the past few weeks, we've been tracking a monarch butterfly caterpillar (Danaus plexippus) that lives on our patio. It appeared magically on a milkweed (Asclepias incarnata) we were growing and has since progressed through several stages called "instars." At each stage, the caterpillar molts and grows a little larger and wigglier. Its nibblings have also become more pronounced.
While it may seem that life for the monarch caterpillar is a simple all-you-can-eat milkweed buffet, it is actually fraught with danger. The milkweed does not appreciate being nibbled on and has adapted defenses to thwart our munching monarch. Small canals called "laticifers" extend throughout the milkweed's stems and leaves. The laticifers are under pressure and store sticky white latex. If the plant is damaged, latex oozes out. This material congeals on the surface of a leaf and can glue a larvae to the spot. The latex is also laced with potent toxins called "cardenolides" that can make a caterpillar go catatonic if ingested.
As a first instar, tiny monarchs are in serious danger of being glued to death, but as the caterpillar grows it develops structures that allow it to tolerate cardenolides. It even begins to store the chemicals in its body, eventually making the adult butterfly toxic to any hungry birds. Older fifth instars also develop a behavior called "sabotage." Before feeding on a leaf, the caterpillar uses its pincers to crush and cut the "petiole," the part of a leaf connecting it to the stem. This depressurizes the laticifers, reducing the amount of latex pumped into the leaf and ingested by the caterpillar.
If all goes well, the monarch will survive its larvae-stage lunch, pupate into a lovely, jewel-like chrysalis for several days and emerge as a brilliant, well-nourished butterfly. Our caterpillar is currently at the pupa stage and our milkweed, though lightly-chewed upon, is doing well.
Brian Rutter, PhD, is the cofounder of Thing in a Pot Productions and a postdoctoral researcher in plant biology at Indiana University. Subscribe to our newsletter to receive our “Things About Things – Odd Facts About Plants” and video production tips in your inbox every month!
Works Cited:
Brower, L. P., & Moffitt, C. M. (1974). Palatability dynamics of cardenolides in the monarch butterfly. Nature, 249(5454), 280-283.
Dussourd, D. E. (1999). Behavioral sabotage of plant defense: do vein cuts and trenches reduce insect exposure to exudate?. Journal of Insect Behavior, 12(4), 501-515.
Dussourd, D. E. (2017). Behavioral sabotage of plant defenses by insect folivores. Annual review of entomology, 62, 15-34.
