So the cuckoo wasps have lost or "lost" their sting, but the general trend at least among bees seems to be selection for stronger and bigger stings for nest parasites (e.g. Psithyrus)
. Interestingly, Michener (2000) says that bees with the most reduced sting apparatus are Dioxyini, which are parasitic...
Michener also says that some Andrenidae have reductions in their sting and accompanying organs.
Perhaps the telescopic ovipositor (and their ability to roll into tight ball*) have enabled / forced the reduction of the sting in the chrysidid lineage?
[Edit: * this part has to be false, because "lower" chrysids like Amiseginae and Cleptinae cannot(?) roll into ball and still have their stings atrophied(?). Perhaps the ballrolling ability evolved in Chrysidinae, because they coudn't anymore use their sting but had to do something to deter the pissed nest owners they faced at host nests...]