After mating, C. punctorium females build silken retreats on top of grass stalks, in which they construct a single eggsac and guard the hatching offspring till they die in late autumn. The guarding female will attack intruders ferociously (Bellmann 2006). Nevertheless, we found numerous eggsacs to be infected by larvae of a parasitic dipteran that consume all spider eggs before hatching. Infection with these parasites thus leads to a complete reproductive failure of the female. We screened nine native populations in Italy, and ten expanding populations in the Baltic States and northeastern Germany for the presence and abundance of this parasite. Moreover, we gathered information from arachnologists working in the respective areas. To screen for parasites, 10–20 female retreats per site were opened and eggsacs inspected for parasitic larvae. Some larvae were taken to the laboratory where COI barcodes were generated for identification of the species using standard arthropod barcoding primers (Folmer et al. 1994). PCR and sequencing were conducted as described above. The resulting sequences were blasted against the GenBank database, to identify related sequences.