Forest Practices Code - Terminal Weevils Guidebook

[Terminal Weevils Guidebook Table of Contents]

Management strategies

The Forest Practices Code Act and Operational Planning Regulations declared that holders of operational plans must conduct forest health evaluations. The evaluation consists of:

The following sections describe the methods and rationales for conducting risk assessments, pest incidence surveys and management strategies to reduce the risk of damage caused by terminal weevils.

Spruce weevil

Hazard and risk rating system

This section describes methods for identifying forest ecosystems susceptible to potential spruce weevil infestations. First, field observations and survey data are used to delineate known areas of infestation. However, drainages which have no recorded history of weevil damage are not accounted for in this process. A number of different surveys collect weevil attack incidence to calibrate hazard ratings. Refer to the section "Survey and assessment" for the survey procedures.

A second method of identifying stand susceptibility or hazard is based on the developmental requirements of the weevil. A minimum of 888 degree-days (accumulated heat from days above a minimum threshold temperature of 7.2C) are required for developmental completion from egg to adult in coastal weevils. Interior spruce weevil require 785 degree-days. The theoretical distribution of weevil populations can be determined using climate and elevation data, and that can then be used to rate hazard for spruce weevil damage at the landscape level before forested areas are developed.

This method is limited to the identification of hazard over broad geographic areas, such as biogeoclimatic subzones. Climatic hazard ratings have been developed for most of the forest regions. Within each region, ongoing ground truthing is done to verify and improve upon the delineation of the hazard zones. Contact the regional entomologist to obtain a current hazard map for the region you are interested in.

The calculation of degree-days for a period is done by taking the sum of

for each day, where Tmax and Tmin are the daily maximum and minimum air temperatures (C) at 1.5 m in the shade, and 7.2 is the minimum threshold temperature. If a value less than zero is obtained, it is set to zero before summing each day in the month.

A risk-rating system has not been formally developed for the spruce weevil. As a temporary guideline, spruce grown on sites in high-hazard areas where weevils have been noted within 1 km are considered to be at risk. At present, there is no accurate way to predict the severity or duration of weevil damage that a plantation may suffer through its early stages of growth.

Management strategies for stand establishment

Spruce weevil damage has been most severe when plantations with a high proportion of spruce have been established within high-hazard ecosystems. The identification of these highly susceptible sites during the development of the silviculture prescription and prior to planting will greatly reduce the impact of weevil on wood quality.

In the Prince Rupert Forest Region, the Establishment to Free Growing Guidebook recommends planting spruce in certain ecosystem associations where it is not the preferred crop species (e.g., in the CWH vm2), but not as pure stands. The weevil risk warnings in the tree species selection tables are applied in a very general manner and may be improved with localized data.

In the Vancouver Forest Region, a more detailed hazard rating has been developed for specific subzones within the region. For each hazard level, species selection recommendations have been provided. In low-hazard areas, Sitka spruce can be planted in accordance with normal species selection guidelines. Caution should be exercised in medium-hazard areas. Spruce should be planted conservatively; only up to 20% of the stand composition should be spruce, provided that minimum stocking levels of acceptable species will be achieved. In high-hazard areas, there is a high probability that spruce will be severely infested with weevil and will not be acceptable at free growing. Choose alternative species to plant, and limit spruce to a maximum of 10% of the total stocking.

For unaccessed ecosystems or drainages lacking weevil survey data, the accumulated-heat-based hazard rating system, if available, should be consulted. Avoid planting high proportions of spruce in those areas meeting or exceeding the degree-day thresholds (i.e., 888 degree-days for Sitka spruce, 785 degree-days for interior spruce). Contact the regional or district forest health staff for assistance in interpreting the hazard rating system.

The same concepts can be applied for managing weevil in interior spruce. For example, certain subzones within the SBS and ICH ecosystems are high hazard, and where possible mixed species stands should be encouraged. Certain seedlots are very susceptible to weevil attack and damage when planted outside their range. Therefore care should be taken to follow the seed transfer guidelines in the Seed and Vegetative Material Guidebook.

Management strategies for established stands

Survey and assessment

Weevil attack rates vary with stand age and height. It is recommended that weevil damage surveys be delayed until the stand is at least 15 years old, for both the interior and coast, in order to detect the peak annual attack intensity.

Surveys specifically recording spruce weevil damage are conducted with two objectives in mind:

  1. to assess attack levels in a watershed or subzone in order to update the hazard rating and species selection guidelines,
  2. to determine whether weevil damage has exceeded the thresholds set for treatments to minimize the weevil's impact.

Each objective requires different levels of precision and varying survey intensities. Hazard-rating surveys may use low-precision aerial and ground surveys, while treatment threshold surveys require high precision because costly treatments may be prescribed as a result. Treatments affected by the outcome of a weevil survey are:

These treatments and their thresholds for decision are described below.

Risk-rating surveys

Various ground and aerial surveys may be used to validate climate-based hazard ratings, since only a gross estimate of current attack incidence is required. Observations and plot data from silviculture surveys, the Canadian Forest Service's Forest Insect and Disease Survey (FIDS) pests of young stands surveys, multi-pest surveys, the survey for pest incidence (see Forest Health Surveys Guidebook), and specific aerial and ground weevil surveys (see below) may provide useful (and with the latter two surveys, statistically valid) information. Along with the current percentage and total of attacked trees, the ecosystem classification, stand age and species composition should be recorded. An indicator of overstorey brush density should also be noted.

A master map and file containing spruce weevil hazard and risk zones should be maintained in each district office by forest health staff. An updated hazard rating should be issued to districts, licensees and silviculture contractors whenever significant changes are noted.

Aerial surveys

Aerial surveys are effective in determining the current attack rates of plantations. The aerial survey method described below estimates current attack rates within 5% with a 95% level of confidence. As many as 15 plantations can be surveyed in a day from a helicopter at less than half the cost of ground surveys.

This method is useful when a large number of plantations need to be surveyed, and plantations that are obviously above or below the 20% current attack threshold can be ruled out from further ground surveys. If the current attack rates fall in the 10 to 20% range, then additional passes with the helicopter should be done, and the plantations should be scheduled for a subsequent ground survey (see below).

The aerial survey procedure is as follows.

  1. A helicopter must be used, with two experienced observers.

  2. Two passes are made over each plantation. The first is to determine if the plantation has brush and where it is. The brushed area should be omitted from aerial survey. This pass should also view whether snags will inhibit the more detailed pass and whether a high component of subalpine fir (similar in appearance to spruce) is present in the stand. If snags or fir pose significant problems for safety or accuracy, the stand should not be surveyed from the air.

  3. The second pass is conducted at the lowest height that may be flown safely. Five hundred trees should be assessed, with each observer assessing about 250 trees. Each observer should have two hand-held tally counters one for counting the unattacked spruce (no current attack) and the other for counting the total current attacks. The helicopter should fly through the middle of the block at about 15 knots. If 500 trees cannot be assessed on this pass, then another pass should be made either perpendicular to the first or at some other angle as allowed by terrain. An attempt should be made not to recount previously tallied trees.

  4. To save time, if 500 trees have not been counted in the air land the aircraft at a convenient landing spot, and calculate the proportion of attacked trees. Then calculate the sample size required to obtain a 95% confidence limit for the estimate, using the formula below. If more samples are required, make one or two more passes until the target sample size is reached.

Figure 6. Decision chart for assessing the feasibility of rehabilitatiing spruce stands heavily damaged by spruce weevil.

Then calculate the sample size required to obtain 95% confidence limit for the estimate, using the formula belaow. If more samples are required, make one or two more passess until the target sample size is reached.

This calculation should only take a few minutes. The sample size formula for obtaining 95% confidence limits for the proportion of attacked spruce is: where:

P = best estimate of the true proportion (use the proportion calculated from the existing tally, or as a default use 0.5)

E = the precision with which P is estimated (always use 0.5)

N = total number of units in the population (total block area X stems per ha).


The first strip obtains an estimate of 29 attacked trees out of 110 trees sampled therefore giving a proportion of 0.264 (=P). The block is 20 hectares at 1000 stems per ha (N = 20 x 1000). The total number of samples required to tighten the confidence limits to 5% (= E) would be:

Thus an additional (307 - 110 =) 197 trees should be sampled.