Forest Practices Code - Terminal Weevils Guidebook

[Terminal Weevils Guidebook Table of Contents]


Ground surveys

As with the aerial surveys, ground surveys more accurately assess the levels of weevil attack when management decisions require data more precise than from silviculture or other less rigorous surveys. During the free growing survey, candidate stands are usually identified for more intensive survey but other surveys or observations that estimate current attack incidence can also be used. Those stands with >10% current attack, as noted in these surveys, should be targeted for intensive survey if there is a management concern. If attack levels exceed a specific threshold, certain actions may have to be considered (e.g., for stand tending, delay or avoid brushing in stands with >10% current attack) (see Figure 6). Use the more formal weevil survey described below if detailed information is required (e.g., for species conversion decisions).

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

Intensive spruce weevil survey procedures

The purpose of the survey is to establish incidence, not distribution, of weevil damage. Only spruce greater than or equal to 1m in height should be assessed for current and past weevil attacks.

  1. Run 5 m wide (2.5 m on each side of the centre line) strip transects at 100 m intervals.
  2. Where current (less than or equal to 3 years old) regeneration data is not available, establish 3.99 m radius stocking plots. Locate plots at 100 m intervals on transect lines, and tally all acceptable dominant and co-dominant trees by species. Minimum sampling intensity should be 0.25%.
  3. Calculate the mean infestation level from the transect lines as follows:

In addition to the attack and stocking data above, species conversion decisions require information on the degree of height suppression caused by repeated weevil attacks (described in the next section). To capture the additional data:

  1. Make visual estimates of total tree height on a minimum of 30 attacked and 30 unattacked spruce along the transect lines.
  2. From the transects, tally all spruce with significant forks and unacceptable crooks and classify them into two height classes: >5 m and less than or greater than 5 m.

At the free growing stage, trees with crooks, forks or infested leaders are considered unacceptable (refer to Appendix D, Free Growing Damage Standards for British Columbia, in the appropriate Establishment to Free Growing Guidebook).

Stand tending

Under certain conditions, spruce weevil damage may limit stand tending activities.

Spacing (pre-commercial thinning)

There is evidence that densely grown spruce are less affected by spruce weevil than are spruce grown at wider spacings. Pre-commercial thinning in high-hazard ecosystems may lead to increased weevil damage if it is done before the weevil population has peaked in the stand. A trade-off between larger diameters and losses due to weevil damage has to be made. Thinning will improve the overall wood quality in damaged stands by increasing radial growth and removing trees with poor form. Improving radial increment also reduces the severity of defects and permits larger piece sizes, which are less affected by the greater number and size of knots resulting from severe weevil attack. Thin only after attack rates decline, which may mean delaying thinning for 15 to 20 years after the first free growing survey is completed.

Currently, the only decision-making tool available on an experimental basis is the combined tree and stand simulator (TASS)/spruce weevil attack trial (SWAT) model. TASS simulates spruce stands grown under different spacings and stand-tending regimes. SWAT provides the timber recovery data that will produce an estimate of defect rate, log lengths and total volume produced under different weevil attack scenarios. The model has only been calibrated for a few stand types. Spruce weevil hazard does not influence commercial thinning prescriptions.

Pruning

Pruning at an early stage will help improve wood quality in damaged stands by reducing the effects of shortened internodes that concentrate knots following repeated years of attack. Pruning may increase stand value but will not influence volume production or future attack incidence. Pruning may also be beneficial by eliminating subordinate branches from former forks that are large, persistent and produce large knots. Because spruce bark produces epicormic branches when it is exposed to sunlight, pruning should only be done when crown closure has occurred.

Brushing and weeding

Brushing and weeding of infested stands may lead to increased weevil damage, especially if the competing vegetation is providing shade to spruce leaders. Overstorey or side-shading also seem to reduce weevil attack rates. Brush can significantly hamper conifer growth, so the impact of the weevil should be weighed against that of the brush on overall height growth. A decision-making tool – a modified WinTIPSY program that incorporates the effects of brush and weevil attack – is currently under development for interior spruce.

In the interim, a general guideline for the central interior that relates weevil damage, site index and brush complexes follows. For medium to high site index sites with brush species complexes that form overstorey canopies (e.g., aspen, cottonwood, etc.), it will probably always be more beneficial to remove the brush and live with the weevil damage. In this situation, brush hampers spruce growth more than would any increase in weevil damage after brush removal. On medium and high sites with brush species that only form low-lying canopies like willow and alder, the effect of brush on spruce growth is at least equal to the detrimental effects of the weevil after brush removal. Therefore, brushing is not recommended in this situation.

A similar guideline for coastal spruce does not exist, as the relationship among brush, site index and weevil damage has not been as well studied as in the central interior. Consult the regional entomologist for site-specific recommendations on delaying brushing and weeding. In the absence of assistance, unless the spruce is severely threatened by over-topping vegetation, delay or avoid brushing in stands with greater than 10% current attack recorded during free growing surveys. The stand should be re-assessed when it reaches 15 years to determine if weevil damage rates have increased to 20%. If not, brushing then may be considered.

Fertilization

In areas of high weevil hazard, fertilization of young spruce stands may lead to increased levels of weevil infestation. Adult weevils prefer large, thick leaders. Fertilization causes increased leader growth, both in length and diameter (phloem thickness), and therefore presents a more favourable environment for developing weevil broods.

Species conversion (rehabilitation)

Under certain stand conditions, the best course of action is to rehabilitate the stand or strata; that is, to remove all the unacceptable conifers and brush species, prepare the site and replant. Clumps of other species and undamaged or lightly damaged spruce may be left untreated. As this is an extremely expensive activity, all factors must be considered to determine if this decision is economically justifiable (Figures 6, 7). Very few situations require stand conversion, since there is strong evidence that in a well-stocked stand the spruce eventually reach a height where it becomes less susceptible to weevil attack and resume normal growth. However, where stocking is poor, weevil damage may provide the incentive to rehabilitate the stands.

All of the following conditions must be met in order to recommend rehabilitation (Figure 6).

  1. The stand is at least 5 m in average height, a free growing survey completed earlier has shown that the stand is near or below minimum stocking, and all spruce stems are required to reach the minimum stocking standard.
  2. An intensive weevil survey has been completed (see "Intensive spruce weevil survey procedures") that assesses and records the following conditions of the spruce stocking.
  3. Other resource uses, especially wildlife, do not place a high value on the stand in its present condition.

Figure 7. Definition of stagnated height growth.

Direct control options

Although there are a number of techniques that will control damage by spruce weevil, most are either too labor-intensive, not cost-effective, not generally available or acceptable (e.g., insecticides), or require further testing and refinement. Thus no current techniques are available to directly control damage caused by this insect pest. The following is a description of the status of various direct control methods.

Leader clipping

Manual clipping of infested leaders prior to emergence of adult weevils is not considered a viable control option. In practice, it is too labor-intensive to be considered feasible, and results from past operations have been questionable. Clipping must be carried out very thoroughly up to three times a year and, because adult weevils can live for four years, it must be continued for at least five consecutive years. Also, weevil immigration from neighboring populations may extend the required treatment period. It is difficult, physically demanding and highly unpleasant work. Leader clipping combined with parasite augmentation has been tried on an experimental basis and has not shown noticeable reduction in attack levels by spruce weevil. This technique has never been implemented successfully at an operational scale in B.C.

Stem-injected insecticides

Injected systemic insecticides will provide two or more years of control and pose little threat to the environment. This technique is currently under development but is presently very labor-intensive.

Resistant stock

The most promising control prospect is the development of resistant spruce stock. Individual genetically resistant spruce trees have been identified and are currently being tested and propagated to produce planting stock for reforestation. It will be a number of years before the effectiveness of this approach is known.

Lodgepole terminal weevil

Hazard and risk rating system

Young pines are considered at risk when grown on sites within high hazard ecosystems such as the IDF dk and dm and various subzones within the MS, SBS and ESSF zones. It is not possible to accurately predict the severity or duration of weevil infestation that a young stand may suffer over time. Field observations and survey data are needed to delineate areas of infestation in order to identify forest ecosystems susceptible to the successful development and spread of lodgepole terminal weevil.

The hazard rating method presented in this section is a subjective system based on the probability of weevil attack and its impact. This section discusses site and stand parameters that could influence weevil attack, and the optimal timing of treatments when lodgepole terminal weevil is present.

The parameters used for hazard rating lodgepole terminal weevil are ecosystem classification, stand density (stems per ha) and age. An accompanying risk assessment combines the probability of attack in given hazard scenarios with an estimate of expected damage, given certain levels of risk.

Ecosystems in the Kamloops Forest Region are assigned hazard for P. terminalis as follows.

Stands spaced at an early age in the IDF and MS zones are most at risk to weevil attack, and attacked trees have a high probability of developing poor form. The potential for damage is increased when stands are spaced at 8 to 12 years to very low densities. In the MS zone, if a stand is spaced to below 1000 stems per ha at 10 years, up to two-thirds of the trees in the stand are at risk to weevil attack. At low densities (i.e., 3.7 m inter-tree spacing), attack rates may be as high as three attacks per tree by 25 years.

Lodgepole terminal weevil causes several stem defects, some height loss and a branchy tree form. Height loss combined with a defect such as a crook, fork or staghead decreases both volume and quality of the wood. Delaying spacing until the base of the live crown has lifted above the top of the first log can improve final log quality, particularly in the presence of lodgepole terminal weevil. This delay might extend the rotation age slightly, but the extension should be more than offset by the increased value of the harvested logs.

Stand age plays an important part in assessing hazard. Stands are susceptible to lodgepole terminal weevil beginning at five and six years of age when 1.0 to 2.0 m in height, with attacks being most common in 10- to 25-year-old stands. If weevil attack is occurring in stands five to ten years old, even at rates as low as 2–3% annually, the risk of weevils will increase with age, especially if stem density is decreased. Density could be reduced to a target level in stages, thus lessening the impact of the weevil and removing deformed trees in the process.

Defects caused by weevil attack can reasonably be assessed by the third growing season following attack. By the fifth growing season (in accordance with free growing guidelines) the tree has determined the orientation of the laterals, and only minor changes will occur in the future. Therefore, if stands are initially left at high densities and are brought down to target density in two to three entries, trees attacked more than three growing seasons prior to the spacing and which only exhibit creases could be left. Conversely, trees attacked three or more seasons prior to spacing but which bear major defects, such as crooks, forks or stagheads, should be removed.

Management strategies for stand establishment

Lodgepole pine terminal weevil damage has been most severe in naturally regenerated stands (within high hazard ecosystems), following fire, that have been spaced to low densities at an early age. However, many planted stands have also suffered high levels of attack. In planted stands, three factors could be influencing the level of damage incurred by the lodgepole pine terminal weevil.

More information is needed on the resistance of pine to the weevil. It is important to determine the weevil infestation hazard when developing the silviculture prescription.

Where possible in high-hazard ecosystems, species mixes should be promoted. Mixes of pine with spruce, larch, fir or balsam should be considered. Higher densities should also be promoted through the early stages of stand development in high-hazard ecosystems.

Timing of juvenile spacing in lodgepole pine, particularly in naturally regenerated stands, is critical. Spacing early in the life of a stand can prevent stagnation and promote rapid growth. However, in the presence of lodgepole terminal weevil, early spacing can increase the percent of trees attacked and decrease stem quality. If current attack levels exceed 10%, spacing should be modified (leave 2000 to 2400 sph) or delayed until trees are >5 m in height or the stand is about 25 years. Early spacing may influence the branching nature of lodgepole pines, and pruning following the spacing treatment may be needed to increase stem quality.

Management strategies for established stands

Survey and assessment

Stands to be surveyed must be selected carefully to record or estimate the maximum average attack rate, since weevil attack varies with stand age and height. Since there may be differences in weevil population growth rates, it is recommended that weevil damage surveys to detect the peak attack intensity be delayed until the stand is at least 15 years old.

Surveys specifically for recording lodgepole terminal weevil damage are conducted for two reasons:

  1. to assess attack levels over a watershed or ecosystem (zone/subzone) to update the hazard rating and species selection guidelines
  2. to determine if weevil damage has exceeded the thresholds set for spacing or pruning to minimize the weevil's impact.

Hazard rating surveys

Observations and plot data from silviculture surveys, surveys for pest incidence, FIDS pests of young stands surveys, multi-pest surveys (see Forest Health Surveys Guidebook) or a more specific survey for the lodgepole terminal weevil (described below) may provide useful information. Along with the current percentage and total of attacked trees (current and old attacks), the ecosystem classification, stand age and species composition should also be recorded. An indication of tree form (defects) should be noted.

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

Lodgepole terminal weevil survey procedures

Ground surveys accurately assess the levels of weevil attack when management decisions require data more precise than from silviculture or other less rigorous surveys and observations. During the free growing survey, candidate stands are usually identified for more intensive survey, but other surveys or observations that estimate current attack incidence can also be used. Those stands with >10% current attack should be targeted for intensive survey if there is a management concern. The survey procedure is similar to that described for the spruce weevil (see the section "Survey and assessment") with the following adjustments.

At the free growing stage, trees with crooks, forks or infested leaders are considered unacceptable (refer to Appendix D, Free Growing Damage Standards for British Columbia in the Establishment to Free Growing Guidebook). Lodgepole terminal weevil damage is usually compounded when stem rusts are a problem in an opening. Western gall rust and comandra blister rust in particular can accentuate problems caused by lodgepole terminal weevil. These pests should not be considered in isolation, because treatments to lessen the impact of one pest could exacerbate the impact of another.

Stand tending

Under certain conditions, lodgepole terminal weevil damage may limit stand tending activities.

Spacing (pre-commercial thinning)

Stand density influences the percentage of stems attacked and the severity of defect caused by lodgepole terminal weevil attack. Therefore, if critical levels of weevil attack are noticed when assessing stands for spacing, spacing should be delayed or modified. Stands in the IDF and MS with >10% current weevil attack should maintain densities of 1800 sph or higher (preferably 2000–2400 sph) until the stand reaches 5 m in height.

There is evidence that densely grown lodgepole pine are less affected by weevil than lodgepole pine grown at wider spacing. Spacing in high-hazard ecosystems may lead to increased weevil damage if done before the weevil population has peaked in the stand. A trade-off between larger diameters and losses due to weevil damage has to be made. Overall wood quality in damaged stands will be improved by spacing, pruning and removing trees with poor form. Space only when attack rates are <10% annual attack, which may mean delaying spacing for five to ten years after the first free growing survey is completed.

Pruning

Pruning, in conjunction with spacing, may help improve wood quality in weevil-damaged stands by reducing the effects of concentrated knots and decreasing the branchy nature of open-grown lodgepole pine.

Control options

Although there are a number of techniques that may lessen damage by lodgepole terminal weevil, most are either too labor-intensive, not cost-effective, not generally available or acceptable (e.g., insecticides), or require further testing and refinement. Thus there are no currently recommended techniques to directly control this insect pest.

Leader clipping

The manual clipping of infested leaders prior to the emergence of adult weevils works in theory. However, it is too labor-intensive to be considered operational. Clipping must be carried out very thoroughly on an annual basis for a minimum of three consecutive years, because adult weevils can live for three or more years and continually re-attack clipped stands.

Leader clipping for lodgepole terminal weevil requires very precise timing to be effective. Most commonly, attacked leaders fade in the summer following attack. Then, depending on weather and site, the fade occurs at different rates. Therefore, there is a relatively short period (June–July) between the time that fading leaders are visible and the time of adult emergence. Leader clipping will improve the quality of the attacked tree if all but one dominant lateral are clipped at the time infested leaders are removed.


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