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Defoliator Management Guidebook Table of Contents]

Management principles for defoliators

Management of the forest resource to reduce the impact of defoliating insects is an ongoing process of evaluating forest stand and site conditions. The objective of defoliator management is to minimize the impact of defoliators and the risk to forest resources in conjunction with stated management objectives for an area. The potential for defoliator outbreaks and subsequent damage is critical and must be considered during all planning processes and management strategies to address defoliators should be incorporated into higher-level plans as well as forest development plans.

Short-term direct control measures are taken only when defoliator populations reach proportions that threaten forest management objectives or human health. Long-term management strategies, however, can reduce the risk of defoliator damage as well as improve the health and productivity of forest ecosystems. There are five steps to integrating the evaluation of stand, site, and insect populations in order to create plans and prescriptions (Fig. 6). The steps are as follows:

  1. landscape level hazard and risk assessment (forest development plans)
  2. aerial and ground surveys to detect defoliator incidence for plans and prescriptions
  3. predictive sampling methods for developing silviculture prescriptions
  4. stand hazard and risk assessment (predictive and other sampling)
  5. monitoring.

The first step should be done prior to preparing five-year development plans. Landscape level hazard and risk assessment is largely a mapping exercise that incorporates historical data on defoliator outbreaks and site information. Most regional Forest Service offices will have historical maps and information available for the major defoliating insects. The second step is done on an annual basis by the Forest Service, in conjunction with the licensees. Ground surveys are undertaken when aerial surveys detect defoliation or predictive tools indicate a building defoliator population. Predictive sampling methods, step 3, are done in high to moderate hazard ecosystems prior to preparing a silviculture prescription, or when aerial surveys and historical data indicate an impending outbreak. Step 4 is done at the stand level when preparing prescriptions, and when stand treatments are planned. Monitoring, step 5, should be done on a continuous basis (annual).

Landscape and stand level hazard and risk assessment

Hazard and risk assessments must be incorporated into forest development plans and silviculture and stand management prescriptions. Prescriptions must adequately address the current and potential impact and dynamics of defoliators in terms of stand and site ecology. Hazard and risk assessment is particularly important when planning for silviculture treatments such as thinning. If a stand is in a high hazard area and at high risk, then the planning process should identify these risks and apply the appropriate strategy to address the defoliator problem. Two levels of stand and risk assessments are required: (1) a landscape level hazard and risk assessment, and (2) a stand risk assessment. The landscape level hazard and risk assessment considers two key factors:

  1. hazard (or susceptibility) of

  2. risk in a planning area, polygon, or block.

Both of these factors have been considered and are compiled into tables for the five major defoliators. Tables 14, 18, 24, 5 and 31describe hazard at the stand and landscape level. Each table identifies hazard by:

Hazard and risk assessments provide a reasonable level of guidance as to what to expect on various sites. Hazard rating of stands is used for establishing priorities to undertake surveys, treatments, and for developing silviculture prescriptions. In general, factors contributing to stand hazard or susceptibility include:

Stand risk refers to the proximity of a current outbreak and the likelihood that the insects will infest a particular stand. Stand risk can be determined by the density, extent, and proximity of a defoliator population, as well as through predictive sampling methods which will estimate the coming years' population and expected defoliation. Future stand risk can be determined by:

  1. expected frequency and periodicity of outbreaks in a given location

  2. predictive sampling to give a short-term estimate of stand risk.

Distribution maps show historic patterns of defoliator outbreaks (Figures 1 to 5). Areas that have a history of defoliator activity, which may support more than one defoliator species (complexes), and have relatively short time periods between defoliator outbreaks, can be considered as high risk. Stand level risk assessment should consider:

  1. Outbreak history

  2. Determine stage of outbreak

  3. Population and defoliation predictions.

Risk increases with increasing historical occurrence of outbreaks. Stands within areas of frequent historic outbreaks, having a high periodicity, and which satisfy most of the high hazard site and stand parameters listed in Tables 14, 18, 24, 25 and 31 are rated as having high hazard and risk.

Tree and stand impact

Defoliation of trees may result in the following types of damage:

Defoliator impact is a function of severity and duration of defoliation, the species and age of trees attacked, tree spacing, and site quality. Figures 7 and 8 illustrate two methods of estimating overall and current defoliation of attacked trees respectively.

Understorey trees (seedling to pole-size) suffer heaviest defoliation when infested overstorey trees are nearby, as in multi-storied stands, partial cuttings, or the edges of clearcuts that border adjacent, unlogged stands. Cumulative damage to a stand may have an effect on a variety of resources.

On timber resources:

On non-timber resources:


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