AbstractHymenoepimecis is a genus of Darwin wasps in the Polysphincta group of genera (Hymenoptera: Ichneumonidae: Pimplinae) known as ectoparasitoids of a broad spectrum of spiders. The parasitoid induces production of a web known as cocoon web, which provides shelter and support for the wasp pupa. In this study, we describe for the first time the interaction between Hymenoepimecis castilloi Pádua & Sääksjärvi (Hymenoptera: Ichneumonidae) and its host spider Leucauge mariana (Taczanowski) (Araneae: Tetragnathidae) in the Colombian Andes, provide new records of wasp genus distribution, and described the behavioral modifications induced in the spider. Web modifications occurred in the webs of both solitary and aggregated individuals. Adhesive spirals were lacking, and webs were connected to vegetation by multiple threads in all cocoon webs, which was not seen attached to webs of non-parasitized spiders. All parasitoid cocoons were observed hanging on a vertical line in the hub of the cocoon web. As previously described for other species, we believe that this modified web design results in increased web strength and favors parasitoid development during the pupal stage.