One of the many natural wonders of our region is the pawpaw tree, a delightful native resident of shady bottomlands along the mighty Potomac and Patuxent Rivers. A walk amongst these winsome understory trees sets one to wondering why their luxuriant green leaves go virtually unmolested by leaf-eating insects and vertebrates during the growing season. Even the ravenous plague of white-tailed deer seems to shun these plants. Pawpaw has evolved a clever defense in a noxious group of chemicals called annonaceous acetogenins. These bioactive compounds, found in leaves and bark, likely make them unpalatable to hordes of hungry herbivores. In addition to nasty metabolic effects, acetogenins are known to produce a potent emetic response in vertebrates.
Ah, but herbivorous insects often discover ways to deal with defenses thrown at them by plants. In previous episodes we learned how monarch caterpillars turned the tables on milkweeds and used defensive compounds produced by milkweeds for their own defense against predators. A similar story holds for the zebra swallowtail butterfly. Sophisticated chemical analysis revealed that zebra swallowtail caterpillars and adults contained annonaceous acetogenins similar to those found in pawpaws. Scientists believe that these compounds originate in the leaves of pawpaw, are stored in the tissues of caterpillars as they eat leaves, and then passed along to the adult butterfly when the caterpillar pupates. The presence of acetogenins likely helps protect both the beautiful butterflies and their larvae from the beaks and teeth of hungry predators.
Take a moment for a walk along the river and enjoy the pawpaw trees, which will soon produce delicious fruit. Be sure to keep an eye open for zebra swallowtails and spend a few moments searching pawpaw leaves for magnificent zebra caterpillars.
Zebras cannot live on nectar alone. Muddy puddles on the banks of the Potomac provide minerals and other nutrients to sustain life.
“Chemical Defense in the Zebra Swallowtail Butterfly, Eurytides marcellus, Involving Annonaceous Acetogenins” by John M. Martin, Stephen R. Madigosky, Zhe-ming Gu, Dawei Zhou, Jinn Wu, and Jerry L. McLaughlin was consulted in preparation for this episode.