Pine bark beetles feed on trees which have weakened natural defenses. Trees have weakened defenses when they reach the end of their natural life span, or when poor growing conditions weaken them. The trees in many fire areas suffer from overcrowding, perhaps the result of good growing conditions for the first 30-40 years of their lives. Eventually the trees get big enough to crowd each other too much, and most of them die. Beetles move in at some point during the decline. Seems like a natural process to me.
I had been given to believe that tree’s defense against bark beetles is to increase their sap production, to flush them out.
But, during a drought, they can’t produce enough sap, and the beetles win.