Thiacloprid is a neonicotinoid systemic insecticide, the other ingredient appears to be a fungicide (or preservative). Did you spray all the plants with it, including the dormants? Does it say anything on the label about a surfactant (spreader-sticker)?
By what are they growing in I mean it doesn't, from the pictures, look like unamended garden soil, it looks too coarse.
A pH of 7 can still be too high for some daylilies. Whether that has any involvement at all in what's going on now I don't know.
The pest damage looks to be either slugs or cutworms (or some other kind of caterpillar). If there's a slime trail it is slugs or snails. If there is no slime trail then you need to check for caterpillars which may be curled up in the soil at the base of the plant during the day. If you go out after dark with a flashlight you're more likely to see the culprit.
