Oviposition by the Mountain Alcon Blue butterfly increases with host plant flower number and host ant abundance

Abstract

Butterfly fitness strongly depends on larval development, and selection should thus favour butterflies that can detect high-quality host plants for oviposition. In myrmecophilic butterflies, which need plants for initial larval development, but rely on ants for completing the larval stage, selection may also favour butterflies that can detect plants in the vicinity of host ants. However, the ability of butterflies to detect host ant presence is still under debate.In this paper, we study the factors that determine oviposition of the specialist butterfly Phengaris alcon rebeli on its obligate host plant Gentiana asclepiadea. The factors considered include gentian traits, vegetation descriptors and host ant abundance (P. alcon larvae can only complete their lifecycle when nursed by Myrmica host ants). Using (generalised) linear models, we found that both oviposition probability and egg number increased with increased gentian flower number per branch and with increased abundance of the host ant Myrmica scabrinodis. In turn, M. scabrinodis abundance increased with gentian flower number per branch and decreased with gentian density. Path analysis confirmed that the positive effects of M. scabrinodis abundance on oviposition probability and egg number are not spurious, i.e., they did not arise simply because flower number per branch has a positive effect on host ant abundance and oviposition. While not proving causality, our findings suggest that oviposition preference of P. alcon butterflies is affected both by host plant traits and by abundance of the host ant. We discuss our findings within the context of the long-standing debate on whether or not ant-dependent oviposition occurs in the genus Phengaris, and the potential underlying mechanisms.