Some old nesting blocks from the bee hotel now function as a stand for a large tub turned into a flower pot and as a potential nesting place for digger wasps. Earlier I had seen an Ectemnius female disappear under one of the blocks and standing guard to record her appearance once she would show herself.
While waiting I noticed a spider-wasp female landing on the block and disappear under the tub. A little later she landed again. Now she had my attention.
At the next landing it was clear she was carrying something between her jaws. Based on the black colour this could be Auplopus carbonarius , a wasp that builds her brood cells using clay, in which case the transported material was probably mud. After a number of mud-flights she stayed away for a long period and my attention refocussed back to the Ectemius.
Then she suddenly appeared and landed on an empty flashlight that I had thought to use to look under the tub. It was clear she was now carrying a spider, Philodromus dispar (det: Frank van de Putte).
After some time the wasp appeared again and took of for more mud-flights. The mud is used to first closed the now filled brood-cell and than start construction of the next cell .
As I was processing the photographs I noticed the chalcid wasp hitching a ride on the spider. That wasp is present in all pictures so it hitched the ride on purpose with spider-wasp and was taken by the spider-wasp into the nest. Which rises the question rises who she is aiming for: the spider or the spider-wasp? I have not been able to find information on this.
I’ve received some information through the Waarneming forum concerning the genus Chaenotetrastichus. Searching for more information it turns out that this genus contains species that are known to be a parasite on Auplopus [3,4,5,8]. Specifically C. semiflavus [3,4,5,6] and C. grangeri [6,7] are mentioned in relation to Auplopus carbonarius, the first as a gregareous parasitoid . At least the species C. grangeri is known to occur in the Netherlands .
Having said all that, although the colouration seems to match the two Chaenotetrastichus species the wasp is not well articulated enough in the photos to make some kind of identification and it remains a guess to which family or genus it belongs.
References1 Nederlands Soortenregister
2 KURCZEWSKI, Frank E.; O'BRIEN, Mark. Auplopus carbonarius, a Palearctic spider wasp, extends its range to Michigan (Hymenoptera: Pompilidae). The Great Lakes Entomologist, 1991, 24.3: 10.
3 VIDAL, S. Redescription and first record from Europe of the North American species Chaenotetrastichus semiflavus Girault, 1917 (Hymenoptera, Chalcidoidea: Eulophidae). Entomofauna, 1996, 17: 149-152.
4 La Salle, John. (1994). North American genera of Tetrastichinae (Hymenoptera: Eulophidae). Journal of Natural History. 28. 10.1080/00222939400770091.
5 ASKEW, R. R., et al. Chaenotetrastichus semiflavus (Girault)(Hymenoptera: Eulophidae, Tetrastichinae), a genus and species new to Britain. Entomologist's Monthly Magazine, 2014, 150.1796-98: 43-45.
6 Noyes, J.S. 2021 Universal Chalcidoidea Database. World Wide Web electronic publication. http://www.nhm.ac.uk/chalcidoids
7 Soortenbank.nl 2021
8 Marinho, Diego & Vivallo, Felipe. (2020). Notes on the bionomy of two spider wasp species in an urban forest fragment in Brazil. Papéis Avulsos de Zoologia (São Paulo). 60. e20206055. 10.11606/1807-0205/2020.60.55.