Chemical Facilitation of a Naturally Occurring Host Shift by Papilio Machaon Butterflies (papilionidae)

Host shifts by herbivorous insects have contributed substantially to current patterns of association between insects and plants. Despite their evolutionary and agricultural interest, however, the plant traits that predispose insects to colonize some plants instead of others are poorly understood. To examine whether ancestral and novel hosts share similar chemical oviposition stimulants that would facilitate a host shift, we investigated a wellsubstantiated host shift within the Papilio machaon group of swallowtail butterflies. Papilio machaon aliaska uses three plant species as host plants. Cnidium cnidiifolium belongs to the family Apiaceae, the ancestral host-plant family of the P. machaon group. Artemisia arctica and Petasites frigidus, by contrast, belong to the family Asteraceae and were colonized relatively recently by this group of butterflies. Papilio machaon oregonius, a close relative of P. m. aliaska, feeds and oviposits exclusively on Artemisia dracunculus, also in the Asteraceae. We made polar and nonpolar extracts of all four host plants, conducted bioassays with P. m. aliaska and P. m. oregonius females to test for oviposition stimulants, and found that the polar extracts were the most active. Using high-performance liquid chromatography, we separated the polar extracts into three fractions and again conducted bioassays with P. m. aliaska and P. m. oregonius females. The fraction containing hydroxycinnamic acid (HCA) derivatives was the only active fraction for all plant species. We further separated the HCA fraction and found two sub-fractions that were active in all of the host-plant species. Co-chromatography indicated that several major constituents of the active fractions are shared between ancestral and novel hosts. In a secondary series of experiments, we investigated chemical fractions of a non-host plant, Artemisia frigida, and identified fractions of the A. frigida extract that contained oviposition deterrents and a fraction that contained oviposition stimulants for P. m. aliaska females. We conclude that the similarity of stimulant profiles in ancestral and novel host plants is consistent with the hypothesis that plant chemistry has played a role in the establishment of this host shift.