The Pattern of Social Parasitism in Maculinea teleius Butterfly Is Driven by the Size and Spatial Distribution of the Host Ant Nests

Simple Summary Maculinea butterflies are endangered social parasites of Myrmica ants. In late summer, caterpillars abandon their foodplants and wait for a Myrmica worker to retrieve and carry them into the nest. Here the caterpillars spend 11 months consuming the ant brood or being fed by workers; then they pupate early in summer, and in one month the adults emerge. Our study aimed to assess the spatial relationship between nests parasitized by Maculinea teleius and those unparasitized and the factors influencing the parasite presence inside host nests. We searched for Ma. teleius caterpillars in ant nests in autumn, during the initial larval development, and in the following late spring. Unsurprisingly, we found a substantial decrease in the proportion of parasitized nests from autumn to late spring. The biggest Myrmica nests adopted a higher number of parasites, but mid-size nests provided the best trade-off between competition and resource availability, leading to high parasite survival observed in spring. The spatial distribution of parasitized nests in autumn was uniform, while the colonies in which Ma. teleius survived until pupation were grouped. Overall, our results suggest that host colonies' features and spatial relationships should be considered when trying to preserve these rare butterflies. The parasitic relationship between Maculinea butterflies and Myrmica ants has been extensively studied but little information is available on the spatial occurrence of Maculinea larvae. We searched for the presence of Maculinea teleius in 211 ant nests at two sites in two crucial phases of its life cycle, i.e., in autumn, during the initial larval development, and in the following late spring, before pupation. We assessed variations in the proportion of infested nests and factors correlated with spatial distributions of parasites in Myrmica colonies. The parasitism rate in autumn was very high (similar to 50% of infestation rate) but decreased in the following spring. The most important factor explaining parasite occurrence in both seasons was the nest size. Further factors, such as the presence of other parasites, the Myrmica species or the site, concurred to explain the differential survival of Ma. teleius until the final development. Irrespective of the host nest distribution, the parasite distribution changed from even in autumn to clumped in late spring. Our work showed that the survival of Ma. teleius is correlated with colony features but also with the nest spatial distribution, which therefore should be taken into consideration in conservation strategies aiming at preserving these endangered species.