Assuming that the holes are small (3mm or less), it may be Shot Hole Borers - these tend to only affect trees already in poor health, and those growing in poor conditions. Chemical treatment is difficult because most of the activity takes place underneath the bark, where there will be a network of tunnels, sometimes even into the heartwood. If its on the main trunk, nothing you can do other than improve the growing conditions by feeding, mulching with humus rich materials and keeping watered during dry spells. If its on branches, these should be removed where possible.