Try wrapping corrugated cardboard around the tree trunk. The grubs will congregate there and can be destroyed. Chickens are another way of eradicating the problem as they consume the larvae. Pherome traps are also available through organic supply places. Then, of course, there are a variety of chemical sprays if that's the way you want to go but most are quite toxic to a range of organisms beside the pests - including to us.