Origins of the regional feeding abilities in the tiger swallowtail butterfly: ecological monophagy and the Papilio glaucus australis subspecies in Florida

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Abstract
The Highlands County, Florida populations of the eastern tiger swallowtail butterfly, Papilio glaucus L., represent a putative subspecies, P. g. australis Maynard. The only foodplant ever reported in southern half of Florida is sweetbay, Magnolia virginiana L. (Magnoliaceae). In fact, there are apparently no other reported foodplants for the polyphagous tiger swallowtail species which exist in southern Florida. These ecologically monophagous P. g. australis populations were studied in order to determine whether or not significant behavioral and biochemical adaptations in the larvae are detectable as a result of the ecological specialization on sweetbay. These results suggest an emphatic “yes”. On sweetbay, neonate larval survival of these Florida populations is the best of any other population tested from across North America. In addition, penultimate instar larval growth rates are significantly faster on sweet bay than any other foodplant tested. Furthermore, these biochemical adaptations to sweetbay by P. g. australis in Florida are accompanied by a significantly poorer neonate survival and growth performance on northern foodplants (most notably the Salicaceae and Betulaceae). Reciprocal inabilities of the northern subspecies, P. g. canadensis, and the western P. rutulus to utilize sweetbay are described, as well as the reciprocal inabilities of two congeneric and sympatric (Florida) species, Papilio palamedes and Papilio troilus, that have specialized on another “bay” (redbay, Persea borbonia; of the Lauraceae). It remains uncertain to what degree such apparent negative genetic correlations in foodplant adaptations represent a driving force in the formation of host races or species, especially since so little is known about oviposition preferences of Papilio glaucus across North America. In Florida we have an apparently strict ecological monophagy coupled with a degree of reproductive isolation that has resulted in its tentative classification as a subspecies, P. g. australis. The role of foodplants in the speciation process can be significant even for P. glaucus, the most polyphagous of all 563 species of world Papilionidae. It remains uncertain whether the significant differences in detoxication and biochemical processing abilities observed in this study were evolved in parapatry or allopatry. Disjunct populations of P. alexiares in Mexico retain some ability to detoxify sweetbay, as do some individuals in most P. glaucus glaucus populations tested from ten different states north of Florida.