The biology of parasitoids in natural ecosystems remain very poorly studied, though they are key species for their functioning. Here we focused on Phobocampe confusa, a Nymphalini specialist, responsible for high mortality rates in charismatic butterﬂy species in Europe (genus Aglais). We studied its ecology and genetic structure in connection with those of its host butterﬂies in Sweden. To this aim, we gathered data from 428 P. confusa individuals reared from 6094 butterﬂy larvae(ofA.urticae,A.io,andintwooccasionsofAraschnialevana)collectedovertwoyears(2017and 2018) and across 19 sites distributed along a 500 km latitudinal gradient. We found that P. confusa is widely distributed along the latitudinal gradient. Its distribution seems constrained over time by the phenology of its hosts. The large variation in climatic conditions between sampling years explains the decrease in phenological overlap between P. confusa and its hosts in 2018 and the 33.5% decrease in the number of butterﬂy larvae infected. At least in this study, P. confusa seems to favour A.urticae as host. While it parasitized nests of A.urticae and A.io equally, the proportion of larvae parasitizedissigniﬁcantlyhigherforA.urticae. Atthelandscapescale,P.confusaisalmostexclusively found in vegetated open land and near deciduous forests, whereas artiﬁcial habitats are negatively correlated with the likelihood of a nest to be parasitized. The genetic analyses on 89 adult P. confusa and 87 adultA.urticaeusing CO1 and AFLP markers reveal a low genetic diversity inP.confusaand a lack of genetic structure in both species, at the scale of our sampling. Further genetic studies using high-resolution genomics tools will be required to better understand the population genetic structure of P.confusa, its biotic interactions with its hosts, and ultimately the stability and the functioning of natural ecosystems.
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