When I dug into the soil, I found a colony of dozens of these black beetles, about 1/2 inch long. Are these beneficial, or do they eat plant parts? I hit them with Azamax and Spinosad, but they didn't even flinch
I have some pics, but they were a little blurry.

Thanks!

I don't know for sure but I think it could be a ground beetle, of which there are a number of species. Generally considered beneficial by eating garden pests.

That's good to hear. Thank you for the ID, Sue!

Turns out that these are black vine root weevils. They have been feasting on my rhododendron. They leave a unique pattern of notches in the leaves, and their larvae (ugly grubs) eat the roots. During the day, they hide under planters and under the mulch. I'm trying diatomaceous earth first, but they might need a pyrethrin drench...or worse...

Are you sure that's what the beetle in your initial picture was? Yes the damage you mention sounds like black vine weevil but in your picture the pronotum looks a different shape and smooth rather than rough, the antennae don't look elbowed (perhaps the specimen was damaged) and the head looks too wide with prominent "eyes" behind the antennae insertion:

Black vine weevil picture:
https://bugguide.net/node/view...

Hi Sue, I will see if I can get a pick of the bugs around my rhododendron. Might be different than the first picture that I submitted. Difficult to tell, at night, lit by a flashlight.