Blister beetle. Depending on the species, they have different feeding habits. Most are herbivores and have a peaking order of plant choices. Yours apparently don't have celery at the top of their list. They will mate and the adults will vanish as quickly as they appeared. Chemical control is almost economically impossible. Soil drenches are much more effective and economical although hand picking or netting is the best. Watch out when handling as they can cause blistering (thus the name) to sensitive skin.