In turf (for grubs etc.) the residual for imidacloprid is several months. Whether that's related to the concentration or something, I don't know. There's an article here about the environmental fate of imidacloprid:
http://www.cdpr.ca.gov/docs/em...
I remember also reading that it can cause a proliferation of spider mites.
An interesting comparison of the toxicity of pesticides to one specific aphid species and their natural predators is here, and includes both imidacloprid and acephate, plus a few others:
https://www.researchgate.net/p...
Several different species of aphid can affect daylilies and how much variation in susceptibity to specific pesticides might relate to individual species I don't know. We can't use most of these "chemical" pesticides in home gardens here in Ontario any more under a provincial cosmetic pesticide ban.
