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.
We have just noticed another tree in the line which is on the point of falling - totally loose in the soil despite the top being fine. I wonder if they buggered up the roots on that one. We have had so many set backs this year. Gonna go and drink a lot of beer. (Cos I never do that normally.)
To lead a tranquil life, mind your own business and work with your hands.
You frighten me terribly. I would like to go home now. Here, take this tiny ad: