It could be the work of a loop caterpillar, although I would have expected to see bigger "bites". It also could be a couple of different pests working together. I've seen this kind of damage on other plants with thrips and leaf miners and then had a chomper come along and chomp away at the already affected leaves.

Are any other plants affected? Bougainvilleas have the reputation of not being bothered by much in the way of pests, so it could be that this is a "crime of opportunity"... a pest that usually attacks something else may be attacking it. Loop caterpillars are nocturnal feeders so you almost never see them. Thrips and leaf miners are tiny and can be easily overlooked.

YES - grrrr - mine too as you can see. Maybe could be blamed on Grasshoppers, but I'm going to take a flashlight to them tonight to see if I can find any other culprits. Those little black things look like caterpillar poops. Also mine have grown stems that sprout to the heavens, but not much color this year.

Now that picture looks very much the work of a chomper (loop caterpillar).

Go to this UC Davis address for this problem ipm.ucdavis.edu/PMG/GARDEN/PLANTS/INVERT/bougainloop.html