As with the mountain bark beetle, it is not entirely clear why this is happening.

ECONOMIST: Getting warmer

Scientists say forests in the Western U.S. have been increasingly damaged in recent years by invasive insect species such as the bark beetle -- a sign that rising temperatures are having an adverse effect.

CNN: Global warming threatens forests, study says

Officials are planning future beetle surveys and will try to limit the infestation with "girdling, " in which thick strips of bark are removed from around infested ash trees.

WSJ: Mass. creates quarantine zone to contain insect