Sinarachna nigricornis and genus-specific host utilization of Araneus spiders by the genus Sinarachna (Hymenoptera: Ichneumonidae)

The ichneumonid parasitoid Sinarachna nigricornis (Holmgren, 1860) was associated only with Araneus sturmi (Hahn, 1831) in the studied locality. The normal capturing orb web of A. sturmi was always vertical and had a median of 18 radii and 9 spirals in laboratory conditions. When spiders did not build capture webs, they built resting and molting webs which consisted of a few threads between the ends of the provided tree twigs. There were no significant differences in the number of radials and spirals between unparasitized spiders and spiders parasitized by early instar parasitoid larvae. Araneus sturmi under manipulation by S. nigricornis built a modified cocoon web, serving parasitoid larva pupation. S. nigricornis exhibited high plasticity in the architecture of the cocoon web induced by its penultimate instar larva. The most common cocoon web was of 3D architecture with a small central silk platform, from which the wasp cocoon was suspended by its apical end (72% of webs). In 22% of cases, the cocoon web consisted only of a few threads, presumably resembling a molting web. In one case, the manipulated spider built a 3D tangle, though radials and the central hub (typical for the normal capturing web) were also present. The genus specificity of the cocoon web architecture described here, in which the typical cocoon web exhibits a sparse but strong 3D architecture and in which the wasp cocoon is suspended by its apical end at the center, is expected for the genus Sinarachna.