Bark beetles kill pine trees by keeping nutrients from traveling from leaves to the rest of the tree and by keeping water from coming upward from the root systems to the leaves. Dehydration and malnutrition combine to damage and kill the trees in which these beetles establish colonies.
Pine bark beetles eat the inner bark of trees, also known as the phloem tissue. The effect on the tree is similar to what would happen if someone peeled all the bark off, or girdled, the tree. The impact is comparable to using a tourniquet to stanch the transport of nutrients from the leaves inward to the other areas of the tree. This means that sugars and other compounds cannot move to parts of the tree that do not perform their own photosynthesis.
Bark beetles also add a blue stain fungus to the tree, growing into the wood. This fungus inhibits the movement of water from the roots up to the leaves, dehydrating the tree.
One of the first signs that bark beetles have invaded is faded needles, changing from their natural dark green to light green, and then to yellow and red over the course of several months. Dust from boring along the tree bar is also a sign, as are tunnels inside the bark, known as galleries. Trees have pitch as a natural defense, but bark beetles create tubes that go through the pitch.