The parasitic insect Xenos vesparum induces noticeable behavioral and physiological changes-e.g. castration-in its female host, the paper wasp Polistes dominula: parasitized putative workers avoid any colony task and desert the colony to survive in the nearby vegetation, like future queens and males do. In this long-term observational study, we describe the spectacular attraction of parasitized workers towards trumpet creeper bushes (Campsis radicans) in early-summer. Two thirds of all wasps that we sampled on these bushes were parasitized, whereas the parasite prevalence was much lower in our study area and most wasps sampled on other nearby flowering bushes were non-parasitized. First, we describe the occurrence and consistency of this phenomenon across different sites and years. Second, we evaluate the spatial behavior of parasitized wasps on C. radicans bushes, which includes site-fidelity, exploitation and defense of rich extra-floral nectaries on buds and calices. Third, we record two critical steps of the lifecycle of X. vesparum on C. radicans: the parasite's mating and a summer release of parasitic larvae, that can infect larval stages of the host if transported to the host's nest. In a nutshell, C. radicans bushes provide many benefits both to the parasite X. vesparum and to its host: they facilitate the parasite's mating and bivoltine lifecycle, a phenomenon never described before for this parasite, while, at the same time, they provide the wasp host with shelter inside trumpet flowers and extrafloral gland secretions, thus likely enhancing host survival and making it a suitable vector for the infection.
The authors are grateful to Marta Mariotti, Laura Maleci, Claudia Giuliani and Corrado Tani for fruitful discussions on Campsis radicans identification and morphology. We also thank two anonymous reviewers and Francesco Dessı`-Fulgheri for their helpful suggestions; Marco Vannini and Leonardo Dapporto for their support in the statistical analyses; Alessandro Pagnini for his help in the search of Campsis radicans bushes in Tuscany; Rita Cervo, Stefano Turillazzi and the members of the Florence Group for the Study of Social Wasps for their
assistance during this study, both in the field and in the laboratory.