A flavonoid glucoside, phellamurin, regulates differential oviposition on a rutaceous plant,Phellodendron amurense, by two sympatric swallowtail butterflies,Papilio protenor andP. xuthus: The front line of a coevolutionary arms race?
We studied the chemical basis for the differential acceptance of a rutaceous plant.Phellodendron amurense, by ovipositing females of two sympatricPapilio butterflies,P. protenor andP. xuthus, that have a very similar host range.P. amurense, on which larvae of both species perform well, was rejected byP. protenor females but was marginally accepted byP. xuthus, even though water-soluble fraction of this plant elicited significant ovipositional responses from the two species. A prenylated dihydroflavonol glucoside, 3,5,7,4′-tetrahydroxy-8-(3-methylbut-2-enyl)flavanone-7-O-β-glucoside (phellamurin), was identified as a dominant active substance that deters oviposition.P. protenor responded highly sensitively to phellamurin, whereasP. xuthus was less susceptible and was affected conspicuously only at higher concentrations than that found in living plants, thereby accounting for their differential responses toP. amurense. InP. protenor, the deterrent effect of phellamurin was considered to outweigh the activity of coexisting stimulant(s) at the concentrations actually present in the foliage, resulting in avoidance of this plant. The phellamurin content in the foliage amounted to as much as 1.8% of the fresh weight, which is far higher than the average levels of flavonoids present in other major host plants ofPapilio. The dose-response experiments demonstrated that unacceptability ofP. amurense forP. protenor was attributable largely to such a high concentration of phellamurin. These results appear to provide information on possible coevolutionary interactions between herbivores and plants and also on the potential for shifts in host affiliation.