Insects & Diseases

Forests Are Ecosystems

Forest insects and diseases are important components of forest ecosystems in Oregon. These pests not only affect the growth and mortality of individual trees, they also influence the species composition, stand structure, and ecological succession of forest stands. While insects and diseases can be damaging and destroy value in our forests, they also have many important ecological roles, such as recycling nutrients, pollinating plants, and providing food for wildlife. As with many situations in our forests and in life, there can be too much of a good thing.

Problems arising from insects and diseases are often associated with other stressors or tree-weakening agents in our forests – blowdown, low tree vigor due to overcrowding, and moisture stress, to name a few. Older trees are often more prone to diseases than younger trees, as older trees are generally less vigorous, and have had more opportunities through their lives for physical damage from a falling branch or tree. Such primary injuries can create an infection site that leads to secondary damage by an insect or disease.

The vast number of insects and diseases potentially present in our forests makes it impossible to address individual species here. However, a few insects and diseases that have been particularly damaging in recent years include:

  • Western spruce budworm – the most destructive defoliator of grand fir, Douglas-fir, and other conifers in central and northeast Oregon.
  • Mountain pine beetle – the most destructive tree-killing beetle in Oregon, impacting large areas of lodgepole and other pines.
  • Swiss needle cast – a foliage disease specific to Douglas-fir, and most common on the western slopes of the Coast Range in central and northern Oregon.
  • Sudden oak death – a non-native disease with a wide range of hosts (including Douglas-fir), including nursery stock, with tree mortality in Oregon presently isolated to tanoak trees in Curry County.
  • Pine butterfly – a defoliating insect that prefers ponderosa pine, with major outbreak in 2011 in eastern Oregon being the first outbreak in decades.

Causes & Controls

The causes and controls of insect and disease problems are as numerous as the pests themselves. The best advice is up-front prevention. While sometimes forest health problems are outside our control, you can do your part as a small woodland owner by keeping your stand growing vigorously and by minimizing damage to residual trees during thinning operations.

Every year since 1947, aerial surveys have recorded the insects and diseases affecting forests in Oregon (and Washington). These annual surveys are cooperative efforts by the forest health staffs of the Oregon Department of Forestry, Washington Department of Natural Resources, and the USFS Pacific Northwest Region. More information about these annual surveys, including maps and other survey results, may be found on these Oregon Department of Forestry web pages:

Additional Resources for Tree Health:

The Department’s forest health professionals conduct surveys, evaluations, and monitor forest insects and tree diseases. They provide technical advice and training in the use of integrated pest management principles to help professional foresters and landowners meet their management goals and objectives.


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