Defoliator Management Guidebook Table of Contents]

Detection and predictive sampling

Defoliator populations are detected through aerial surveys and ground reconnaissance. Status of defoliator populations and trends can be determined through predictive and population sampling methods.

Stands appear brownish or scorched immediately after a season of defoliation. Initially, defoliation is concentrated at the tips of branches on current foliage, or at the tops of trees. Primary host trees suffer the most defoliation with other tree species being defoliated to a lesser extent when they are intermingled with the preferred host. Trees will be defoliated in clumps (as occurs with initial stages of Douglas-fir tussock moth outbreaks), or over large areas encompassing one or more stands (as with budworms and loopers). Canopy and understorey trees will have chewed foliage and take on a brownish tinge.

Signs of insect defoliators include:

Proper identification of the defoliator species is critical. Other damaging agents can cause discoloration and needle loss which may cause confusion when doing aerial surveys. Ground surveys, most often in the form of a walkthrough, must be done to confirm the causal agent. Patterns of defoliation, elevational ranges, tree species and coloration differences help distinguish defoliators at the stand level (Table 4). Tree level diagnostics include feeding and defoliation patterns on individual trees and insect diagnostics.


This section describes various stand and landscape level survey techniques that can be used to detect and predict defoliator populations. Detection and survey methodologies can be used prior to, or following, harvesting or stand management treatments. It is critical to detect and map defoliated areas annually. The building phase of an outbreak is a particularly critical stage and should be monitored closely. These techniques are essential for the creation of silviculture prescriptions (e.g., planning direct control programs). Detection and survey methodologies pertaining to specific insects are described in greater detail elsewhere in the text.

Prompt detection of defoliators is dependent on annual aerial and ground detection, good predictive sampling procedures, and a knowledge of defoliator outbreak cycles and historical occurrence. Outbreak cycles may be divided into four phases
(Figure 9):

  1. Non-outbreak: in areas of historical defoliation, where no defoliation is noticed in aerial overviews, and insect numbers are low.

  2. Building phase: the time interval when insect populations increase, yet very little or no defoliation is visible from aerial surveys.

  3. Outbreak phase: the time period when defoliating insects are causing noticeable damage.

  4. Declining phase: the time interval when areas of defoliation and insect numbers decline and large amounts of damaged trees are visible.

Steps required for defoliator detection and planning purposes are listed below. Specific survey methods are outlined in individual defoliator sections.

  1. Refer to maps showing historic areas of defoliator outbreaks to determine high hazard stands.

  2. Determine defoliator outbreak periodicity and estimate the next outbreak cycle for your planning horizon.

  3. Perform aerial surveys on an annual basis. When patches of defoliation are mapped, additional rotary wing surveys may be necessary to more clearly define defoliation boundaries and timber types in priority areas.

  4. Confirm the causal agent with ground walkthroughs.

  5. The non-outbreak phase can be referred to as the monitoring period. Continue predictive sampling and monitoring (e.g., pheromone trapping) throughout the non-outbreak phase, but at a lower sampling intensity.

  6. When predictive sampling methodologies indicate building populations in high hazard stands, perform ground walkthroughs in select stands to:

  7. During the outbreak and declining phases, compare mapped current year defoliation to the previous year(s) defoliation. Highlight new, expanding, or declining areas of defoliation. Compare the intensity of defoliation from year to year. Many areas will change in both size and intensity of defoliation.

  8. Perform ground walkthroughs in areas where defoliation has increased or expanded, and in adjacent high hazard stands.

Aerial surveys

Aerial surveys may cover a single drainage or larger planning units. Perform aerial surveys on an annual basis in mid-July, or as soon as defoliator damage is most visible. Defoliation should be sketch-mapped at a scale suitable to management objectives (minimum scale 1:125 000).

Survey all high hazard forest types. Defoliated trees, stands, or hillsides assume a reddish tinge. When defoliation is severe or has occurred over many years, stands may appear grey. Detected defoliation should be broken down into three categories: light, moderate, or severe (Table 5). Ground checks should always be done to verify the defoliator.

Stand management objectives determine threshold levels for further aerial or ground surveys and prescriptions. More refined aerial and ground surveys are needed to determine treatments in priority management areas. Management objectives concerning the following topic areas may designate a stand for high priority follow-up work:


The walkthrough is the first opportunity to verify the causal agent. Walkthroughs are generally conducted during the non-outbreak, building and declining phases of a defoliator cycle. Walkthroughs are conducted in priority areas, high hazard stands, or affected areas that are identified from aerial overview surveys. Defoliator activity is verified by a walkthrough. A predictive sampling survey is then strongly recommended prior to writing a silviculture prescription. Thresholds for management of defoliators are described in species-specific sections.

Stand and tree characteristics that should be noted during walkthroughs include:

Predictive and population sampling

Sampling methodologies for defoliators include predictive and population sampling. Predictive sampling is used to forecast future stand defoliation based on an estimate of the insect population. Monitoring the relative abundance of adults and their offspring (eggs or other overwintering stages) will warn of building populations and forecast expected damage levels within a particular stand. These techniques are used in the year preceding the expected damage and are critical in planning for silviculture treatments, such as thinning or direct control. Population sampling techniques determine the current population of insects based on presence of life stages in samples. Actively feeding early and late instars are sampled to determine the current insect population in a stand. This is done to verify the necessity of, and to assess the efficacy of, direct control programs.

Predictive sampling

Predictive surveys estimate expected defoliation in the coming year. These surveys can be used to forecast stand level defoliation, and will help determine when and where to prescribe stand treatments.

  1. Adult monitoring is an annual exercise that monitors changing adult defoliator populations by counting the average number of moths caught in pheromone-baited traps. Pheromone monitoring systems act as a warning signal for impending outbreaks. When threshold levels are reached, one or more of the other predictive sampling surveys should be done. Douglas-fir tussock moth is the only defoliator that presently has a calibrated pheromone trapping system.

  2. Egg sampling is used to predict stand level defoliation for the following summer. The average number of eggs, or egg masses, present on a given area of foliage is calculated and used to forecast potential population density and damage caused by the five major defoliators. When direct control programs are planned, based on egg sampling results, insect population densities must be verified the following spring and summer prior to the commencement of the control program.

  3. Larval sampling predicts the next season's defoliation levels based upon the number of 2nd instar, overwintering larvae (L2). L2 sampling is very time consuming and requires rearing and washing facilities. It is not a recommended technique except in certain circumstances, for example, when other predictive sampling methodologies were not done and direct control is anticipated.

Table 6 describes when each of the above sampling methodologies should be used and for which defoliators each of the sampling methods apply.

Population sampling

Population sampling estimates the current insect population in the stand and the level of damage that can be expected. These techniques verify population levels (or defoliation predictions) and confirm treatment areas in the spring to early summer, prior to the major feeding period (Table 6). Population sampling is also done during the major feeding period to assess efficacy of control programs. When direct control programs are planned based on egg sampling results, it is recommended that verification of the insect population be done the following spring and summer. Sampling methods include budmining and larval surveys.

  1. Budmining surveys are done in the spring to estimate insect population levels (Table 6). The percentage of buds containing early instar larvae is calculated, giving the estimated population levels in a particular stand. This survey can be done for western and eastern spruce budworms. Budmining surveys can be used to verify defoliation predictions made the previous fall from egg surveys.

  2. Larval sampling is used to describe population trends, potential stand damage, or to evaluate efficacy of insecticide treatments. Calculate the average number of live larvae per given area of foliage, collected at discrete intervals during the feeding cycle of the defoliator.

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