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