The term "bark beetles" refers to numerous varieties of very small beetles which attack weakened or dying conifers. Though they will attack pine, spruce, or fir, in this part of Michigan they are mainly associated with red and white pine.

Healthy conifers can usually withstand bark beetle attacks, often by literally trapping the beetles in a sticky flow of pitch. Trees most at risk are those weakened by over-crowding, drought, saturated soils, or disease. Once a tree is infested, the beetles present in the tree will emit a chemical signal, or pheromone, which attracts more beetles to the tree.

Briefly, female beetles penetrate the bark and feed and reproduce in the cambium, the thin layer of tissue between the bark and the wood. The female lays eggs in the cambium, and as the eggs hatch, the young larvae chew and excavate tunnels. These tunnels in turn stop the flow of water and nutrients up and down the tree, which occurs in the cambium layer.

Trees under attack often fade from the top down. Needle color will turn from bright green to yellow and finally reddish brown. Bark beetles generally kill trees in "pockets" or groups. They are often associated with red pine plantations. However, they can also attack ornamental and windbreak conifers.

The best control for bark beetles is prevention. Healthy trees are able to withstand most bark beetle attacks. Insecticidal sprays will not kill the adult beetles or larvae inside the tree as they are under the bark.

Control measures include:
• Remove dead or dying trees.
• If cut logs are to be used for firewood, strip the bark, or put a tarp over the wood.
• In drought periods, water ornamental trees.
• Utilize ìtrapî logs to attract beetles. Destroy logs when infested.

Picture shows dead red pine in a windbreak. Insert shows close-up of entrance/exit holes in lower 8 feet of trunk.

more info: http://ohioline.osu.edu/hyg-fact/2000/2557.html