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Why do zebra swallowtails eat pawpaw leaves?

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With 132 genera and somewhere around 2,300 species, the Annonaceae or pond apple family are native to lowland tropics. The odd members of this family are the eight species of pawpaws (Asimina) found in warm-temperate and temperate zones of North America. Few insects or grazing animals bother plants in the Annonaceae; the plants contain acetogenins. So far, this class of chemicals has been found only in the pond apple family.

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Netted pawpaw
(Asimina reticulata Shuttleworth ex Chapman)
Plant of the Week 03/15/2004

Acetogenins are relatively new to science. The first acetogenin, uvaricin, was discovered by S.D. Jolad, J.J. Hoffman, K.H. Schram, J.R. Cole, M.S. Tempesta, G.R. Kriek, and R.B. Bates of the University of Arizona in 1982. Uvaricin is a powerful antitumor agent. Since then, three hundred-fifty acetogenins have been discovered with only 35 species of the 2,300 species examined. These chemicals have cytotoxic, antitumor, antimalarial, immunosuppressive, and pesticidal properties. They may well become new drugs and new pesticides. ("Catalytic Antibodies, Synthetic Proteins, and Annonaceous Acetogenins", E. Keinan, F. Grynszpan, et al, Skaggs Institute for Chemical Biology Scientific Report 1999-2000)

In nature, acetogenins serve as antifeedants; they discourage herbivores from eating the plants. Pawpaws are seldom found with leaf damage. Even the netted pawpaw (Asimina reticulata Shuttleworth ex Chapman) growing in a pasture, was left ungrazed when everything else had been eaten by cattle. (See Plant of the Week, March 15, 2004) But the pawpaws serve as the only hosts (larval food)
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Zebra Swallowtail (Euritides Marcellas)
Priscilla Connell
Buy at AllPosters.com Framed Mounted

for the zebra swallowtail.

Like the pawpaws, the zebra swallowtail (Eurytides marcellus [Cramer]) is an oddity. Most of the Eurytides butterflies are found in the tropics. The zebra swallowtail is found as far north as Michigan and New York. The butterfly survives where its major host, Asimina triloba [ah sim' i na tri low' ba], grows. In the southern states, the butterfly also uses the other species of pawpaws.

It was once believed that pawpaw leaves were fibrous, watery, and had little nutrition. Zebra swallowtail larvae were thought adapted to a plant with little food value. But the discovery of acetogenins changed that perspective. John M. Martin, Stephen R. Madigosky, Zhe-ming Gu, Dawei Zhou, Jinn Wu, and Jerry L. McLaughlin tested tissue samples of both the larvae and the adult butterflies. The tissues were found to contain acetogenins and this probably serves as a chemical defense against predators like birds. ("Chemical Defense in the Zebra Swallowtail Butterfly, Eurytides marcellus, Involving Annonaceous Acetogenins", Journal of Natural Products, American Chemical Society, October 21,1998)

Butterflies and Moths of North America, has photographs of the zebra swallowtail (Eurytides marcellus [Cramer]) taken by Paul Opler and additional information about this beautiful butterfly. To view the page, click on the link:

http://www.butterfliesandmoths.org/species?l=1353

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