Phengaris (Maculinea) alcon butterflies deposit their eggs on tall plants with many large buds in the vicinity of Myrmica ants

The survival of eggs and larvae is dependent on the oviposition site selection of their mothers. In obligate myrmecophilic butterflies, both host plant phenology and host ant presence are expected to affect the decision where to deposit eggs. The importance of ant nest presence in the oviposition site selection of Phengaris butterflies is, however, highly debated. We studied oviposition in the largest Phengaris (Maculinea) alcon population in Portugal, exploiting Gentiana pneumonanthe as the host plant and Myrmica aloba as host ant. We collected phenological plant data and recorded the presence and number of eggs on plants with and without Myrmica ants nearby during the flight period of the butterfly. Females oviposited on tall plants with many tall buds, while the presence of host ant nests weakly affected oviposition on plants where the probability of finding ants at close range was high. Moreover, larger plants with many tall buds close to host ant nests received more eggs. A density-dependent shift in oviposition was not found as the proportion of buds not infected with eggs did not differ between plants with or without ants, whereas plant characteristics did have an effect. Tall plants with many large buds were associated with earlier oviposition. Our results suggest that females of P. alcon in Portugal choose gentian plants for oviposition mainly based on plant characteristics whereas the vicinity of ants had a weak effect. Moreover, our study shows that testing the ant-mediated oviposition hypothesis requires baiting ants more than once.