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Forest Insects and Diseases

Forest insects and diseases are serious threats and can have devastating impacts on the long-term health and sustainability of forest ecosystems.

Diseases, such as chestnut blight and Dutch elm disease, and insect pests, such as emerald ash borer, gypsy moth, and hemlock woolly adelgid, already have significantly changed our forest landscapes.

DCNR’s Bureau of Forestry monitors Pennsylvania’s forests for insects and diseases, protecting trees when necessary.

Current Threats to Pennsylvania Forests

Of the pests that affect our commonwealth’s forests, the insects and diseases that have caused the most damage in terms of defoliation and mortality during recent years include:

  • Emerald ash borer
  • Gypsy moth
  • Hemlock woolly adelgid
  • Beech bark disease
  • Oak wilt

Oaks continue to be at risk from gypsy moth defoliation and oak wilt disease, while beech bark disease continues to expand and threaten beech populations.

Threats to oaks and beech are especially important because they are the largest remaining sources of hard mast for wildlife after the demise of the American chestnut.

Additionally, hemlock woolly adelgid, introduced into Pennsylvania in 1967, continues to spread westward and is affecting the eastern hemlock, Pennsylvania’s state tree.

Similarly, the emerald ash borer was detected in Pennsylvania in 2007, and is now found in most of Pennsylvania causing widespread ash mortality.

Monitoring Insects and Diseases in Pennsylvania Forests

DCNR’s Bureau of Forestry has a variety of active surveys and projects to monitor and manage forest insects and diseases.

Each year, the DCNR Bureau of Forestry conducts an aerial survey program to detect and map:

  • Tree dieback
  • Mortality
  • Defoliation
  • Foliage Discoloration

Ground surveys are done to confirm the suspected insect or disease for each mapped area.

The information is used to:

  • Determine the extent of damage for insects and diseases of concern
  • Anticipate future outbreaks
  • Make management recommendationsake management recommendations

The data are provided to the USDA Forest Service for developing regional and national maps of current outbreaks, risk maps, and models to predict future conditions for pests of concern. These resources include:

Factors Contributing to Forest Insect and Disease Infestations and Tree Mortality

As trees age or are stressed by external factors, they become less able to fight off insects and disease-causing pathogens, eventually succumbing to insect infestations and diseases that help finish off the declining tree.

External factors that can stress trees include:

  • Drought
  • Excessive moisture
  • Pollution
  • Abnormal temperatures
  • Wind damage

These trees are ultimately replaced by younger healthier trees growing in the understory (lower vegetation layer that includes young trees, shrubs, and other plants) through the natural regeneration of trees and forests.

While native insects and diseases (bark borer beetles, bark beetles, Armillaria root rot) contribute to the death of old and stressed trees and lead the way to the regeneration of trees and forests, non-native insects and pathogens can dramatically alter this cycle.

Trees have less ability to fight off non-native invaders and they can succumb even when young and healthy.