A new Symmorphus garden species: S. gracilis , the male and the female.
During a period of about a week, a female and at least two males were involved in a luring process where the males seemed to place scent markers on plants to lure the female, or females.
I am studying the literature on this particular topic. A first indication is that some wasps will apply a feromone from their jaw gland and smear it out on the surface with their abdomen. The smearing could be clearly seen, assuming this is the process that I observed. A male would land on a leaf and could be seen wiping the surface with its abdomen left and right. On this picture the wasp can be seen while wiping right.
This process played out on a limited part of the garden and concentrated around a Forsythia plant. The males flew in from the right and would land on a leaf, often the same leafs but not always, moved their abdomen and proceded to another leaf or continued their flight path into a Japanese Rose. Their they would fly between the branches and leafs, sometimes land on one, and disappear out of sight near the back of the plant which is positioned against the open fence around the garden. I assume they exited the garden, turned right and would enter through the fence a few meters further and start the process again.
References1 Nederlands Soortenregister