LARVAL ADAPTATION TO LAURACEOUS HOSTS - GEOGRAPHIC DIVERGENCE IN THE SPICEBUSH SWALLOWTAIL BUTTERFLY

Geographic differences in host plant availability are thought to result in regional adaptation of herbivores to their sympatric host species. Larvae of Papilio troilus, the spicebush swallowtail butterfly, feed only on plants of the Lauraceae. Although this insect occurs throughout the eastern United States, each host species has a distribution smaller than the range of P. troilus. In peninsular Florida, the major host is Persea borbonia; Sassafras albidum and Lindera benzoin are the predominant hosts elsewhere. The hypothesis that P. troilus are better adapted to their sympatric host plants was tested using a split-brood design comparing the larval performance of Florida, Midwest, and hybrid Florida x Midwest full-sib families on Persea, Sassafras, and Lindera. Florida larvae had higher first-instar growth rates and survival on Persea compared to Midwest larvae. In contrast, Midwest larvae had higher total larval growth rates on Sassafras and Lindera relative to Florida larvae. A significant host x geographic strain interaction was also found for pupal mass. Performance of hybrid larvae was intermediate to the parental strains, or equalled the best performing parental strain, depending on the host and performance measure. Regional differentiation in genetically based traits for host use indicates the existence of geographic races in P. troilus.