Koa Looper Caterpillars (Scotorythra paludicola, Geometridae) Have Lower Fitness on Koa (Acacia koa, Fabaceae) True Leaves than on Phyllodes

Abstract

Native plant-herbivore interactions on islands remain understudied due to the widely discussed idea that island plants have weaker defenses than their con- tinental relatives. In Hawaii, the native moth Scotorythra paludicola, a specialist on the native Acacia koa, can undergo outbreaks that defoliate tens of thousands of acres of native koa forest, sometimes leading to massive stand mortality. Such extreme herbivory events are expected to exert strong selection pressure for defense in A. koa. Because mature A. koa trees often re-flush juvenile true leaves after defoliation, we predict that true leaves are better defended against S. paludicola than phyllodes, consistent with the phenomenon of induced resistance. A no-choice bioassay was conducted in the laboratory to compare S. paludicola development on true leaves vs. phyllodes. Consistent with our predictions, caterpillars reared on true leaves had a significantly higher mortality rate and took longer to pupate than caterpillars reared on phyllodes. Additional sources of variation in S. paludicola development included sex, phyllode age (young vs. mature), and host tree identity. Further research is needed to determine the mechanistic traits underlying A. koa resistance to S. paludicola, and to test whether true leaf development does in fact contribute to a reduction in S. paludicola performance and population stability on previously defoliated trees.