Host plant selection and differential survival on two Aristolochia L. species in an insular population of Zerynthia cassandra

Understanding host plant preference and the relative quality of resource provided by co-occurring host plants is a key step to predict butterfly species abundance and responses to environmental changes, and, consequently, to plan management measures. Zerynthia cassandra is an Italian endemic species strongly dependent on the availability of its host plants, Aristolochia rotunda and Aristolochia lutea. The insular population occurring on Elba island (Tuscan Archipelago) is highly threatened, because of limited host plant distribution, small population size and apparent lack of gene flow with the mainland. In 2017, we carried out field surveys and rearing experiments to (i) identify the characteristics of the host plants (vegetative status) and the site characteristics (aspect, irradiation, distance from other patches) correlated with the number of eggs occurring on individual plants, (ii) compare larval growth, food-conversions rate and larval and adult survivorship on the two host plants species. Egg occurrence depends on patch irradiation, the number of leaves and flowers occurring on individual plants and the occurrence of nearby patches. These findings allowed to identify the optimal Aristolochia patch features for egg laying and development. Laboratory rearing success was higher than 50% and although plant species did not show a significant effect on oviposition, we found that larval and adult survival was higher on A. rotunda. Our results suggest habitat management aimed at increasing resource availability for Z. cassandra and possible ex-situ conservation actions aimed at recovering the population in case of potential catastrophic events.