I believed that they did and I have seen fields stuffed with Red and Black Froghoppers and the 'spittle' was everywhere. But having a look for reliable sources I haven't found anything to say specifically that they do.
However, the research is about the Xylella bacterium which I read is spread by xylem feeding insects. Cercopis vulnerata is considered to be a vector for this bacteria, which suggests they are xylem feeders and liable to produce the 'spittle'.
So on balance I would say yes they do.