Oviposition trials of Euphydryas phaeton on Plantago lanceolata

Understanding the circumstances under which insect herbivores will adopt a novel host plant is a longstanding question in basic and applied ecology. Though there has been ample study of regional differences in host preference across numerous insect herbivores, the extent to which intraspecific variation in plant species shape these patterns is relatively less well known. In parts of its range the Baltimore checkerspot Euphydryas phaeton uses English plantain Plantago lanceolata, a novel host plant, in addition to its native host, White turtlehead Chelone galbra. We offered female butterflies from each region the non-native host lpant sourced from both regions and compared their oviposition behavior. The non-native host was near universally rejected by butterflies in the region where only the native plant is used. In the region where both hosts are used, plants from that same region were preferred relative to plants from the other region. This suggests that regional variation in host plant use by an insect depends on intraspecific variation in both species in the interaction.

Methods
Female butterflies in two parts of the Baltimore checkerspots' range (two field sites in Maryland and one site in Massachusetts) were offered White turtlehead to assess their gravidity in the field. Gravid females were offerred English plantain from both regions randomly in sequence, in addition to some naturally growing plants, to understand how acceptance differs based on butterfly and plant origin. See the Methods of the associated manuscript and Fig. 1 for more details.