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The Healthy Forests Initiative and Healthy Forests Restoration Act
Interim Field Guide

Determining the Existence of an Insect or Disease Epidemic

Except for cases of wind throw, blowdown, or ice-storm damage, HFRA Section 102(a)(4) requires the existence of an epidemic on, or adjacent to, NFS or BLM land and the imminent risk that the epidemic will spread. Resource managers need to understand the potential for such insect and disease epidemics to develop and spread.

What Is an Insect or Disease Epidemic? Epidemic refers to populations of damaging insects and pathogens that build up, often rapidly, to injuriously high levels (figure 10). Epidemic is synonymous with outbreak. Ecologically, an outbreak is often an explosive increase in the abundance of a particular species over a relatively short period. For example, Douglas-fir tussock moth populations can increase to tree-killing levels and then subside over a 3- or 4-year period. Other outbreaks, such as dwarf-mistletoe, may take years to increase to damaging levels and can persist for decades.

Aerial photograph of a heavily forested area in which several trees have orange-colored limbs indicative of insect infestations.
Figure 10—Epidemic levels of insects or diseases, such as this southern pine beetle
outbreak on the Sam Houston National Forest in Texas, produce forest conditions that
have all the ingredients leading to a fast-moving, high-intensity catastrophic wildland fire.

Some factors that could be considered when determining whether an epidemic exists include:

  • Current population levels relative to endemic levels

  • Observed rates and extent of population increase and/or spread

  • Species composition of the stand

  • The age and size of the trees in the stand

  • Stand densities or stocking levels

  • Climate and seasonal weather patterns

  • Disturbance events such as wind, snow, and ice storms

Insect or disease epidemics result from vulnerable stand conditions (hazard, see the Glossary) and increasing pest populations (risk, see the Glossary). An understanding of implications of a particular outbreak will come from an evaluation of the interaction of species, forest conditions, and weather-related phenomena, such as extended periods of drought and high winds.

The Field Manager (DOI BLM) or Forest Supervisor (USDA Forest Service) will determine whether an epidemic exists under Section 102(a)(4) of the HFRA after consulting with forest health specialists (entomologists and pathologists) who know the factors that are relevant to such a determination.

Evaluating the Threats Posed to Ecosystem Components or Forest or Rangeland Resources

Factors to consider when evaluating the threat that insect or disease epidemics pose to ecosystem components or forest or rangeland resources include:

  • Forest and stand conditions

  • Pest populations and their rate of increase or decrease

  • Weather-related conditions such as drought

  • Fire

  • Tree damage from a variety of causes

Forest and stand conditions determine the effects of insects or disease. For example, the greatest biological factor affecting bark beetle populations is the availability of food, which is determined by the conditions of their host species within a forest. Attributes of a given stand that influence bark beetle activity include: species composition, the age and size of the trees, and the density of the trees.

Drought stress is caused by prolonged periods of extremely low precipitation that reduce soil moisture below the requirements for trees. Drought stress can predispose trees to insect and disease epidemics by compromising or inhibiting their defense mechanisms. Prolonged periods of drought are associated with mortality caused by root diseases, bark beetles, and woodborers. Increased moisture also can increase the likelihood of infection by pathogens, such as the exotic white pine blister rust, and other pathogens that affect a tree’s foliage.

Fire often kills trees or severely stresses them by injuring their foliage, stem, or root systems. Many species of insects are attracted to trees injured by fires. Bark beetle populations that are active in stands before a fire, combined with susceptible stand conditions, could increase the likelihood of additional tree mortality after a fire. Fire can also indirectly affect the hazard when fire cycles are interrupted, leading to changes in the species composition, density, and structure of a stand, which can affect the incidence and likelihood of spread of many pathogens, such as dwarf mistletoe and root diseases, and increase the hazard to damage by many species of insects, such as the western spruce budworm and Douglas-fir tussock moth.

It is important to identify the potential short- and long-term effects of these events on ecosystem components or forest and rangeland resources so treatments can be developed to reduce harmful effects. Coordination among fuel specialists, ecologists, silviculturists and forest health specialists is important.

Computerized hazard- and risk-rating models are available for several forest insect and disease pests. These models are linked to forest stand development models, such as the Forest Vegetation Simulator and should be used whenever possible to help increase reliability when assessing the spread of insect or disease epidemics. Such assessments should be made by forest health specialists who have professional knowledge of the behavior of insect and disease populations, the factors that contribute to the outbreak, development, and spread of epidemics, and the potential effects of epidemics on ecosystem components.

Forest health specialists should provide expert advice to resource managers on the actions that are available to reduce threats to ecosystem components or forest and rangeland resources.

Effective management strategies for direct and indirect control of insect or disease outbreaks include prevention, suppression, and restoration. Prevention strategies are designed to change the conditions that render forests susceptible to epidemics. Suppression strategies are designed to suppress or control existing populations of insects and pathogens. Restoration strategies reestablish an ecosystem’s ecological integrity so that the ecosystem’s components are functioning and capable of self-renewal.


The analysis and documentation for threats from insects and disease under Section 102(a)(4) of the HFRA are intended to be integrated with the analysis and documentation done under current NEPA guidance and other relevant guidance. This documentation should be included in the NEPA documents normally prepared during project planning, the Decision Records or Records of Decision prepared before project implementation, or in the project file itself.

Insect or disease risk-reduction projects carried out under the HFRA should document the factors considered and the methods used in making determinations. Where possible, the hazards and risks supporting any determination that a “significant threat” exists should be quantified. The short- and long-term effects of proposed treatments and the effects of taking no action should be described as provided for in the Judicial Review section.

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