A little success. Earlier I had sown parsnip and carrot, see here, but the young plant fell vitim to the many snails in the garden. So I’ve bought parsnip plants instead and planted them on two locations. I have removed the other vegetatin and added a thick layer of sand on the soil.
The parsnip plants took root and are blooming with screens of sall yellow-green flowers. And for sure I have observed a gasteruption female from the assectator aggregate drinking nectar from the flowers on a number of occasions already!
The species probably is G. assectator, and I’m planning to catch one for a photo session to determine the species.
References1 Nederlands Soortenregister