The pine beetle kills pine trees by boring into the phloem, which is responsible for the transport of nutrients throughout the tree. The tree responds to the beetle attack by secreting a resin to block the beetles from feeding on the tree. The beetles have developed a symbiotic relationship with the blue stain fungi, which blocks the trees ability to produce the resin. This bluish color can be seen on the sap of infected trees. Because of this, the tree is starved of nutrients. I would imagine that because it takes the longest for nutrients to travel to the top of the tree, that the top dies first. The areas of the tree nearest to the roots would die last.
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