Bark beetles are so-named because the best known species reproduce in the inner bark (living and dead phloem tissues) of trees. Some species, such as the mountain pine beetle (Dendroctonus ponderosae), attack and kill live trees. Most, however, live in dead, weakened, or dying hosts. Bark beetles are ecologically and economically significant. Outbreak species help to renew the forest by killing older trees. Other species aid in the decomposition of dead wood. However, several outbreak-prone species are known as notorious pests.
Bark beetles often attack trees that are already weakened by disease, drought, overcrowding or physical damage. Healthy trees may put up defenses by producing resin or latex, which may contain a number of insecticidal and fungicidal compounds that can kill or injure attacking insects, or simply immobilize and suffocate them with the sticky fluid. Under outbreak conditions, as we are seeing presently in our western forests the sheer number of beetles can however overwhelm the tree's defences, and the results can be disastrous.