Hi info, 'Early Treatment' are the key words. As soon as the adult beetles are seen, spray or dust the lilies with either Permethrin, Pirmiphos-methyl or Bifenthrin.
Good Luck Tbird+
My prized tiger lillies were infested a couple of years ago. I didn't want to spray so just picked off the messy blobs and squashed the grub inside. The beetles gave up and haven't been seen since.
It's a pity such beautiful beetles are so destructive.