There's a longer list of pest possibilities depending on location. On this AHS list check out Lopidea confluenta, Hemerocallis gall midge, tarnished plant bug as well as the more commonly suspected thrips. I've seen daylilies heavily infested with spider mites but not abort buds so I'm not sure if that is typical.
http://www.daylilies.org/ahs_d...
It could also be environmental - nutritional, moisture related, something going on with the roots and so on. Bud drop in daylilies comes in the "who knows", and various possibilites categories.
One clue sometimes might be the colour of the flowers that are being affected.
