It could be a weevil problem. Have a look around the base of the
trees.
Treatment choices if it is weevils include:
1: pyrethrum spray the trunk
2: sticky tape around the base of the trees to catch them
3: a line of
petroleum jelly around the trunk to catch them
4: maybe painting the trunks with a white wash, like done with citrus will do
5: keeping the ground clear - maybe run chooks around to eat them
Hope it's that simple, good luck.