The bees will bore holes wherever rain water will not enter, meaning sides or underneath of wood, never on 'top'. Oil based paint works best. If you use an insecticide, use one that has a residual affect so they carry it 'back to the family'. Pressure treated wood isn't as much as a deterrant as it use to be due to the change in chemicals used to process the wood. If left untreated, woodpeckers will peck at the wood to get the baby bees and/or larva. If the birds don't get them and they 'hatch' out, they return to their nesting places year after year to start the process over again.