Something's eating my Black-eyed Susan
Rudbeckia spp. is commonly called Black-eyed Susan, but I suspect that is not the plant you have. Perennial Black-eyed Susan doesn't have any specific pests that prey on it. You may have Black-eyed Susan vine, which is Thunbergia alata, an annual. These vines are susceptible to whiteflies, which secrete a sticky substance caled "honeydew." Ants love to eat it, so they may be taking care of the problem, albeit slowly. Use Neem Oil Soap for an organic solution to the problem.