Thank you for reaching out to ask2.extension.
 It is difficult to totally prevent these insects, but you can try these methods to manage them:

Start checking lilies and fritillaries in early spring for signs of lily beetles, and remove them before a damaging population develops

Regularly check plants every 2–3 days and manually pick off adult beetles, larvae, and eggs. You can drop them into hot soapy water or crush their shells with your fingers.

You can try insecticidal soaps, neem oil, or spinosad, an insecticide made from soil bacteria. However, be careful not to spray when it's windy or when the flowers are open, as the spray could also harm bees and other pollinators.

Plant resistant varieties Some lilies, like Lilium henryi 'Madame Butterfly', Lilium speciosum 'Uchida', and Lilium 'Black Beauty', are more resistant to lily leaf beetles.

Encourage wildlife Birds, frogs, wasps, and predatory ground beetles eat lily beetle grubs and sometimes adult beetles. You can also try releasing parasitic wasps, which are natural predators that keep lily leaf beetle populations in check.

I hope this will help!