They generally will not eat live plants; they may be scraping algae and other aufwuchs off the wisteria's leaves.
They've definitely cleaned the wisteria off. I used to have some dark algae buildup in the center part of the plant where the otto didn't venture. It' all clean and clear now. I did find a few other resources that said they will clean a plant of algae, but a lot that suggested the algae would just be an appetizer.