Does the head of that insect end in what looks almost like a snout? If so, I think it's a kind of weevil.
As far as I know, the larval stage feeds on plant
roots. That could explain why you hard early signs of damage before you saw the adults covering the
trees. Beneficial nematodes applied to the soil early can target the larval stage of this and many other pest insects.
Diatomaceous earth is pretty much my go to for adult insects on the few occasions I bother fighting it. I don't know if that's the best approach though. I'm sure there are many other options that people here could suggest.