Wasps getting high ??

I have never seen anything like this before. A whole clump of Rosebay Willowherb full of wasps systematically stripping the outer skin. They ripped bits off, which they chewed and then discarded the pulp before moving up to continue with the next bit. I can only surmise that they liked the sap? There was very little flying about but a lot of dedicated chomping.

your suggestions would be welcome.