Insects and disease

These tiny and sneaky threats cause big problems

Insects and disease are an important part of the forest ecosystem.  They are always present at some level in the forest. Many provide food and shelter for other forest residents and are an important part of the forest life cycle.  Problems occur when these insects and diseases reach epidemic or excessive levels and threaten the very system of which they are a part.

Primary insect and disease threats in Idaho

Defoliators – Most defoliators are moths.  In recent years, the tussock moth has caused the most concern affecting 15% of the forested areas on the Boise National Forest.  Defoliators eat the lush, green needles of the tree, weakening it and making it highly vulnerable to other pests and diseases.

Dwarf Mistletoes – These are one of the most recognizable tree diseases because of the large, clumpy growths found on infected trees.  Often called “witches brooms,” these abnormal growths are the result of a mistletoe plant that robs nutrients from a tree.  The primary effect of dwarf mistletoes is growth reduction, but top-kill and death can occur in heavily infected trees.


Bark Beetles – Bark beetles are another threat. They attack weakened trees by boring through the bark to eat the cambium layer.  Depending on the species of beetles, they will attack trees that are weakened by overcrowding, drought, root diseases, defoliation and weather damage.  Perhaps the most serious pest problem facing Idaho forests is the Mountain Pine Bark Beetle.

Root Diseases – Common root diseases are “hidden killers” of the forests because they attack the roots of the tree.  Armillaria root rot is the most widespread root disease in the northwest. Its fungus is generally present in certain trees but remains in check by the normal protection mechanisms of a healthy tree.  However, when a tree is weak and unhealthy, Armillaria can spread rapidly, taking over the tree’s root system and moving up the tree.


Watch a video of a Tussock Moth outbreak in Idaho from the Department of Lands here. 

Read Frequently Asked Questions about the Tussock Moth in Idaho here. 

Learn how Tussock Moths affect humans here. 

Review the Forest Health Highlights in Idaho from the Idaho Department of Lands and the US Forest Service here.

Review previous year’s Forest Health Highlights in Idaho here.

Learn more about insects and disease and gain technical assistance and training, visit the Idaho Department of Land’s website, here.

Read the “Field Guide to Diseases & Insects of the Rocky Mountain Region” here.