I will check on your question.  As I understand it, birch is an understory tree often growing in moist soils. When I lived in Ellensburg, before we had BBB on the west side, the reason BBB was attacking trees is that they were planted in open areas with full sun.  Ellensburg has a high water table so that may have balanced the water loss at the top of the tree.  However, if the water table dropped the tree experienced drought stress making them susceptible to borers.  A stressed tree "smells" different and the beetles are then attracted to and are able to lay eggs. 

As we have experienced more and more drought, bronze birch borer has become a problem on the west side of the mountains. Many tree companies have not been able to improve the tree condition.  Your's must have hit the timing just right.  The key is going to be to ensure the trees don't experience drought stress.  As long as water is reaching all parts of the tree they aren't likely to be drawing in beetles.

However, I will check with some entomologists that have dealt with this insect over the long term and get back to you.