Right now swarms the beetle Apalus bimaculatus on open sandy areas that gravel pits and ditches. It has overwintered in the ground as imago and is now ready to mate. The adult beetle is only seen above ground during a few weeks just after the snow melts in spring, normally from the end of March to the beginning of April. Today I went to the best venue I know, there was full speed, I stopped counting after two hundred!
The larva of Apalus bimaculatus live in the nest of the solitary bee species Colletes cunicularius, who primarily collects pollen on Salix caprea. When it finds a nest it will eat the food stored for the bee larva and supposedly eventually the bee egg or larva itself. It seems to stay as a triangulin larva at least a week and after that it goes to the next larva stage. The pupa will then hatch during autumn and the imago will stay the winter in the bee nest to emerge early next spring to mate.
The species is considered endangered, NT on the Red List.