Abstract
Butterflies of the genus Zerynthia often have different host plants in the genus Aristolochia in the different parts of their distribution range. In Mediterranean France the Southern Festoon reaches its North Western distribution limit, where it generally lays eggs on A. rotunda in either coastal wet meadows or riparian and garrigue habitats in the backcountry. In the study area (Hérault and Gard departments), its distribution range overlaps with that of the Spanish Festoon Zerynthia rumina at its north-eastern limits. The Spanish Festoon lays its eggs on A. pistolochia plants growing in dry garrigues in the backcountry. Three types of field observations were conducted to determine : (1) whether habitat variation influences the rates of egg deposition by the Southern Festoon, (2) the extent to which A. rotunda is the preferential host plant and (3) if both Southern and Spanish Festoons are specialised if they co-occur with both their host plants in proximity. We found that A. rotunda is the main host plant of the Southern Festoon in both habitats where it grows in Mediterranean France although egg laying was observed on A. clematitis and more rarely on A. paucinervis in coastal sites. Finally, in parapatry, Southern and Spanish Festoons exchange their host plants but remain both more specialised on a different host. Despite their protection status, destruction of their habitats by urbanisation and infrastructures are a major threat to these specialised butterflies.