Well, this is an interesting picture.  I really don't know what is going on so will make a guess.  Many solitary wasps, such as this one, feed upon egg masses of other insects.  So my guess, and it is just that, is that this wasp is carrying the egg mass of another insect.  Depositing it upon a plant stem may be just a form of storage.  In any event I doubt that this will harm the plant though you may want to keep an eye on things for a week or two.

Interesting!