Thank you for your question about your coreopsis and insect infestation. Unfortunately, there are at least three different types of insects (plus slugs and snails) that eat coreopsis. We need a photo of the leaves that have been eaten, as well as a photo of the insect before we can give you any definitive answer or solution. Thanks!
attached, please see photo of coreopsis bugs. Thanks for any help you can offer
Thank you for the photo. It confirms that your coreopsis (aka tickseed) is infested with the coreopsis beetle, Calligrapha California. (There are 2 related but different insects with that same name, the other called the Chrysomelid leaf beetle.) The 'it looks like a ladybug/lady beetle' and confirming picture is dispositive. Sadly, however, we have no way to slow down--let alone stop--this insect. It eats when it's in the larval stage and it eats as an adult and it's partial to coreopsis. There is some research indicating that the bt bacteria will kill the larva, but your plants are long past that remedy--as the insects are now adults. I'm sorry we have no solution for you. If you examine your coreopsis next year, and find larva, there are garden-store bt (bacillus thuringiensis) applications that are effective. But by the time they reach 'beetle' status, nothing is effective. Good luck!