Early Stage of Host Range Expansion by a Specialist Herbivore, Euphydryas Phaeton (nymphalidae)1

We examined whether larval and adult behavior, physiology, and chemical defense were altered as a result of host range expansion by the Baltimore checkerspot (Euphydryas phaeton, Nymphalidae) from the native host plant, turtlehead (Cheloneglabra, Scrophulariaceae), to the introduced weed, plantain (Plantago lanceolata, Plantaginaceae). We found that newly hatched larvae from eggs collected from a population using plantain were heavier than those from a population using turtlehead. Nonetheless, both the prediapause and postdiapause larvae derived from the turtlehead population and fed turtlehead in a laboratory experiment gained more mass than those from the plantain population fed plantain. Collections of diapausing larvae from field sites corroborated that pattern. Regardless of the population source (i.e., those using either turtlehead or plantain), postdiapause larvae reared on turtlehead exhibited higher relative growth rate (RGR), efficiency of conversion of ingested food (ECI), and efficiency of conversion of digested food (ECD) than those fed plantain, even though approximate digestibility (AD) and leaf nitrogen concentrations were lower for turtlehead. Regardless of their population of origin, choice tests showed that newly hatched larvae preferred turtlehead. Likewise, adult females reared from larvae collected from both populations oviposited exclusively on turtlehead. Both C. glabra and P. lanceolata contain iridoid glycosides. The iridoid glycoside profile of butterflies reared on these two plants differed, reflecting the differences of the host plants. The shift of some populations of E. phaeton onto plantain is most likely a function of several ecological factors rather than genetic differentiation between populations using turtlehead and plantain.