Ditch lilies are only a form of common daylily, Hemerocallis fulva, and not a real (or 'true') lily at all. Daylilies are prone to a number of insect and disease issues but it is almost impossible to accurately diagnose a plant problem without seeing the plant :-)

I'd suggest you repost this in the Daylilies forum and include photos of the plants focusing on the issues that are causing you concern. btw, daylilies can become overcrowded after a few years and that can decrease flowering. And overcrowded plants will compete for nutrients and moisture as well, which could account for the yellowing foliage. It may be time to divided and discard, relocate or give away the extras.