A number of these small ichneumon wasps Campopleginae sp. (thanks to Hilco Meijer) were involved in a mating ritual on Garlic mustard. The plant probably did not play any special role in this ritual. Eventually tow males and a female remained and the male (left) in the picture was the lucky winner. The couple moved across the leaf top and bottom side, followed by the second male at a distance.
References1 Nederlands Soortenregister