Sex, density dependence, and urbanization level shape host infection by an obligate endoparasite

Anthropogenic changes alter host-parasite dynamics, but the way urbanization influences these relationships remains understudied, despite the diversity in species and transmission modes. We investigated infection of the solitary wasp Isodontia mexicana Saussure, 1867 (Hymenoptera: Sphecidae) by the twisted-wing insect Eupathocera auripedis Pierce, 1911 (Strepsiptera: Xenidae), a rarely documented obligate endoparasite, over a three-year period. We recorded 40 stylopized I. mexicana individuals out of 321 wasps examined, totaling 69 individual E. auripedis, including six found embedded in a single host. Female wasps were larger than males and showed no change in body size with stylopization, whereas stylopized males were significantly smaller than their non-stylopized counterparts. We observed density dependence between host and parasite: wasp abundance, stylopization rates, and the number of strepsipterans per host were all positively correlated. All variables declined significantly along a gradient from low to high urbanization, with tree cover the most important determinant for nesting habitat quality. Although open green space was not directly associated with host or parasite variables, it remains important for I. mexicana, which depends on these areas for tree crickets (Oecanthus spp.) to provision offspring and nectar from asters and mints. Thus, while nesting may be more dependent on forested areas, these highly mobile wasps likely rely on open green spaces for foraging which may serve as interception points for parasites. Further research is needed to better understand the influence of land cover on host-parasite interactions. Our findings highlight the utility of trap nests for improving the study of cryptic interactions, the use of stylopization as a bioindicator, and new insights into urban strepsipteran ecology.