I noticed the eggs early this spring after going to inspect the dead tree limb that they were housed on. They don't infest any plants that I know of as they are predators of soft bodied insects, dining mainly on beetles and caterpillars. I've observed them eating beetles in my garden, flying insects and spiders too.
They're considered beneficial since they will eat bugs like Japanese beetles and larvae that damage plants. But, they're indiscriminate eaters and will eat other beneficials... it's not perfect but I usually leave them alone. They are good at hiding, so maybe that's why you haven't seen any? I think they can be found all over North America.
Nymph and long horned beetle -
Adult