Novel host plant use by a specialist insect depends on geographic variation in both the host and herbivore species

Abstract

Abstract Understanding the circumstances under which insect herbivores will adopt a novel host plant is a longstanding question in basic and applied ecology. While geographic variation in host use can arise through both differences in herbivore preference and in plant quality, there is a tendency to attribute geographic variation to regional differences in herbivore preference alone. This is especially true for herbivores specialized on one or a few plant species. We compared the contribution of variation in herbivore preference versus host plant quality to regional differences in adoption of a non-native host by a highly specialized herbivore, Euphydryas phaeton. In parts of its range, E. phaeton uses only a native host, Chelone glabra, while in others it also uses an introduced host, Plantago lanceolata. We offered female butterflies from each region the non-native host plant sourced from both regions and compared their oviposition behavior. The non-native host was almost universally rejected by butterflies in the region where only the native plant is used. In the region where butterflies use both hosts, females accepted non-native plants from their natal region twice as often as non-native plants from the other region where they are not used. Acceptance differed substantially among individual butterflies within regions but not among plants within regions. Thus, both individual preference and regional differences in both the insect and non-native host contributed to the geographic variation in different ways. These results highlight that, in addition to herbivore preference, regional differences in plant quality may be important drivers of diet breadth.