To lay or not to lay: oviposition of Maculinea arion in relation to Myrmica ant presence and host plant phenology

The choices made by ovipositing female butterflies play a key role in the survival of their offspring and consequently in the persistence of butterfly populations. These choices are even more crucial in the case of obligate myrmecophilous organisms such as Maculinea butterflies with larvae that, after a phytophagous period, need to be adopted by Myrmica ants to complete their life cycle. Because the worker ants’ foraging range is limited, selecting an ’ideal’ oviposition site requires that both the phenological stage of the larval food plant (short-term larval fitness) and the presence of suitable host ants (long-term larval fitness) are taken into account. Whether the female’s selection of a valuable oviposition plant is influenced by the closeness of a Myrmica nest is unclear. We studied the oviposition behaviour of a Maculinea arion population exploiting Origanum vulgare as a host plant. By following females, we collected phenological data on the visited plants that were either ’chosen’ for oviposition or ’avoided’ (flowers were visited and evaluated, but received no eggs), and we assessed the presence of Myrmica ants in the vicinity of each plant. Results suggest that plants are selected by M. arion females on the basis of their bud phenology and the presence of host ants and not of other environmental features. We thus hypothesize the evolution of an adaptive mechanism that affords females of this strictly myrmecophilous butterfly the ability to ensure the long-term survival of their brood by selecting host plants growing near a Myrmica nest.