I'm not sure what's going on here. I assume some symbiotic relationship between the wasp and these bugs (aphids?). Cleaning up? Maybe Ted MacRay could offer an explanation.
Ted reports: The nymphs look like they belong to the genus Aetalion, a primitive family of treehoppers (order Hemiptera) - I can see an adult in the lower left, which helps with the ID. Treehoppers suck plant sap and excrete excess sugars as 'honeydew' - I presume the wasp is there lapping up the sweet sticky stuff. Treehoppers are often guarded by ants, who protect them from predators in exchange for their honeydew.