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Defoliator Management Guidebook Table of Contents]

Western spruce budworm

The western spruce budworm is an important native defoliator of interior Douglas-fir. Other tree species such as the true firs, larch and to a lesser degree, spruce can also be impacted by the budworm. Periodically, population levels reach outbreak proportions in B.C., the most current of which covered 800 000 ha at its peak in 1987, mostly in the southern interior of the province. Outbreaks in susceptible interior forests are thought to be influenced primarily by weather, and therefore fluctuate in an irregular and unpredictable fashion. Budworm outbreaks may be sustained for up to 25 years.

The greatest impact and highest mortality due to budworm is among suppressed and intermediate trees. Factors contributing to stand susceptibility include high stand density, species composition (high Douglas-fir component), stand structure (single versus multi-layered), elevation, aspect and tree vigor (Table 13). Table 14 presents expected volume losses in stands in the southern interior of the province given three different levels of defoliation. Reduction in the 20-year periodic annual increment of the total volume averaged 54.1, 75.0 and 82.3%, for light, moderate, and severe infestations.

Repeated budworm defoliation causes scattered tree mortality over large areas, reduction of growth rates, volume loss and reduced lumber quality. Sustained attack results in complete defoliation in 4 to 5 years. Once an infestation has subsided, defoliated trees take several years to regain a full foliage complement and, therefore, radial growth rates require several years to attain normal growth. Successive years of defoliation in stands may predispose trees to other insects and pathogens. Tree mortality may continue due to root disease, bark beetles, loss of vigor, and other causes even though the infestation has subsided.

Past forestry practices in the interior Douglas-fir types have exacerbated the impact of budworm. Selective harvesting has removed much of the ponderosa pine component, leaving a predominantly Douglas-fir forest. Pine regeneration has been excluded on many sites due to the regenerative features of Douglas-fir. In a more natural scenario, frequent ground fires would kill much of this regeneration, promoting a higher proportion of ponderosa pine. However, improved fire suppression has resulted in dense regeneration of Douglas-fir in some stands, particularly in the suppressed and intermediate layers. This combination of reduced species mix, fire exclusion, multi-layered canopies and dense understorey and intermediate canopies, has created a high hazard environment in terms of the budworm.

Population dynamics

Outbreaks of western spruce budworm are influenced primarily by climate and weather and, therefore, fluctuate in an irregular and unpredictable manner. In stands which are chronically affected by budworm, outbreaks may be sustained for 25 years or longer. Feeding by larvae causes damage to buds, cones, and new foliage. When populations reach very high densities, larvae will also back-feed on the older foliage causing significant damage in one growing season. Understorey trees in multi-layered stands may sustain intense damage due to larval migration from overstorey trees.

By combining the historical occurrence of outbreaks, both in terms of area affected and periodicity of outbreaks, with stand parameters influencing hazard (Table 13), the relative risk in a particular area can be estimated.

Outbreaks may decline through any combination of processes that operate against any one of the life stages. Exceptionally low survival of 2nd instar larvae could be caused by differences in timing between larval emergence from hibernation and budburst, late frosts that destroy both the new foliage and the budworm, foliage depletion through previous feeding, and prolonged rainfall during moth dispersal. Natural mortality factors include parasites, predators (mainly ants and birds), pathogens, and starvation.

Predictive sampling techniques and population monitoring

Population monitoring should be done in all high hazard ecosystems (Table 5). When harvesting or other silvicultural treatments are planned in high hazard ecosystems, both population monitoring and predictive surveys should be done. Stand treatments such as thinning, or methods of partial cutting, should not be done during a budworm outbreak unless direct control is planned for the treatment area. When sampling results indicate moderate or greater insect population levels resulting in moderate to severe defoliation, direct control should be considered.

Egg mass surveys

Egg mass surveys tend to underestimate populations in the building phase of a budworm outbreak and overestimate the population in the declining phase of an outbreak. Defoliation forecasts based on egg mass sampling during the outbreak phase are generally reliable.

  1. Western spruce budworm egg mass sampling should take place in late summer or fall (mid-August through October).

  2. Collect samples at 30–50 m intervals from the mid-crown of mature, well-foliated Douglas-fir trees. Understorey or severely defoliated trees should not be selected for sampling.

  3. Two 45 cm branch tips from the north and south aspects of well-foliated Douglas-fir trees are randomly collected from each of 10 trees per site.

  4. Width and length of each branch sample are measured and foliar area calculated:

  5. The egg masses are counted, using a magnifying lamp or under ultra-violet lighting (egg masses fluouresce), and the information is recorded. [2] Do not confuse the old and new egg masses.

  6. All samples must be processed within one or two days of collection. Prolonged storage will result in sample desiccation and needle loss from the branches, thus rendering the samples useless.

  7. Calculate the number of new egg masses per 10 m2 of foliage.

  8. Use the following table to predict potential defoliation at each site.

The formula to arrive at predicted defoliation is as follows:

Predicted defoliation by the western spruce budworm based on the number of new egg masses/10 m2 foliage as listed below.

Figure 11 illustrates the field form that can be used to tally and calculate the number of egg masses found at each sampling location.

L2 surveys

L2 sampling is done to predict western spruce budworm population levels, and defoliation severity in the coming season. If egg mass sampling was not done, and if defoliation predictions are required in advance of larval activity (i.e., if insecticide spraying is anticipated), L2 surveys can be done in early spring (mid-February to early April). This technique requires rearing facilities and is more time consuming than the other predictive methodologies described for western spruce budworm and therefore is not recommended unless absolutely necessary. L2 sampling procedures

  1. Collect samples at 50–100 m intervals from the mid-crown of mature, well-foliated Douglas-fir trees. Understorey or severely defoliated trees should not be selected for sampling.

  2. Randomly collect two 45 cm branch tips from the north and south aspects of well-foliated Douglas-fir trees from each of 5–10 trees.

  3. Mark sample sites on a forest cover map (1:15 000–1:50 000 scale) for future reference.

  4. Place samples, separated by site, in a rearing facility (see methods below).

  5. Count the larvae on a periodic basis until emergence is complete. If it is necessary to store the branches prior to beginning the rearing process, keep the foliage at 0ē C to prevent larval emergence.

  6. Record the following information for each site:

Suggestions for rearing L2 larvae

Method #1: Use a plastic or cardboard ice-cream bucket painted black on the outside to reduce light transmission and with the top closed. Glue a plastic petri-dish bottom to the bucket lid. Drill a hole (0.5 cm diameter) through the petri-dish and the lid. Place the petri-dish lid on its bottom section which then acts as a catchment for larvae. All buckets are placed on a holding rack with the petri-dishes facing a light source. Emerging larvae are attracted to the light, crawl up into the petri-dish and are collected there. Record number of larvae collected each day. Branches should be left in the bucket for about 20 days; at which time, they should be visually examined for any remaining budworm larvae.

Method #2: Place branch samples in a double paper grocery bag, double fold the top, staple securely, and cover all seams with wide masking tape. Mist the bag lightly once a day for a 10-day period keeping the environment humid but not wet. Bags must be kept at room temperature (20ēC) with at least 10 hours of strong light per day. After 10 days, begin checking every two days for larval activity. As larvae emerge, remove from rearing facility and record number removed. Repeat this process for another 10 days or until no additional larvae are observed.

Predicted defoliation by the western spruce budworm is based on the number of larvae/m2 foliage as listed below:

Budmining surveys

Budmining surveys predict defoliation and are intended to verify fall egg mass survey results. This survey may indicate larval populations that warrant treatment that were not detected in the egg mass surveys, or will highlight declining populations due to overwinter mortality factors.

  1. Collect samples at random within the designated sample area at 25–50 metre intervals ensuring that all elevations are covered.

  2. Collect samples from the mid-crown of mature, representative Douglas-fir trees. Understorey or severely defoliated trees should not be selected for sampling. A 45 cm branch tip from each of the north and south aspects of selected trees are collected from 3-tree clusters (3 trees growing in close proximity, but not directly adjacent to each other). Approximately five to ten 3-tree clusters should be sampled at each site.

  3. Assess all buds on branch tips (6 branches per sample), and record:

  4. Calculate the average per cent buds infested per sample per site. Based on the scale given below, a prediction of defoliation can be made.

Prediction of defoliation by the western spruce budworm can be based on the scale below:

Larval sampling

Larval sampling is used to predict population trends and to evaluate efficacy of insecticide treatments. Larval sampling is usually done prior to, and at regular intervals following, a direct control program (pre- and post-spray sampling). Sample trees for pre- and post-spray assessment are established within and outside of designated spray blocks. To determine population trends, the same sample trees must be visited at each sample time. Areas chosen for sampling outside of the treatment block(s) must have budworm population levels comparable to the treatment area (based on budmining assessments). The total number of sample trees is decided upon by the number of spray blocks and hectares to be treated (Table 17).

Pre- and post-spray sampling procedure

  1. Collect samples at 50–100 m intervals from the mid-crown of mature, representative Douglas-fir trees. Understorey or severely defoliated trees should not be selected for sampling.

  2. 45 cm branch tips from each of the north and south aspects of selected sample trees are collected.

  3. Width and total length of each branch sample are measured and foliar area calculated (Fig. 11):

  4. Count all larvae on the branch, categorizing by instar (Fig. 12, Table 15):

  5. Rate current year defoliation of each sample using Fettes scale (Fig. 8).

  6. Pre-spray sampling should be done one day prior to treatment. The pre-spray sample may have to be redone if the treatment has not occurred within 3 days of the initial pre-spray sampling date.

  7. Post-spray sampling is done at weekly intervals until pupation occurs (>50% of the insects sampled are pupae). Data is recorded as per (4) above.

  8. Per cent mortality and Abbott's corrected mortality can then be calculated.

Per cent mortality:

The per cent mortality calculates the per cent of insects that have died in the time interval between the pre-spray sample date and post-spray sample date(s). The per cent mortality must be determined for treated and untreated (check) areas. Per cent mortality is calculated as follows:

Abbott's corrected mortality:

Abbott's corrected mortality calculates the mortality attributed to the insecticide treatment by comparing the mortality in the treatment block to the natural mortality seen in the untreated check areas. Abbott's corrected mortality is calculated as follows:

Long-term strategies

Long-term management strategies are stand manipulation tactics including:

Alternate species

Alternate species are less susceptible to budworm feeding. Even-age silviculture systems (clearcut, seed tree, shelterwood) can be prescribed in appropriate ecosystems in conjuction with the promotion or planting of alternate species. Species that are less susceptible to western spruce budworm include: western larch, lodgepole and ponderosa pine, and spruce. All other species commonly grown in B.C. (excluding Douglas-fir and true firs) can be considered non-hosts.

Species mixes

Single- or multi-layered stands composed of mixed species are likely to sustain outbreaks shorter in duration and of lesser intensity. In ecosystems where uneven-age, or multi-layered stand management is the desired silviculture system, or is the only option, species mosaics should be promoted where possible. Silviculture and treatment prescriptions that promote species mixes include: patch cut and plant, partial cutting (selective cuts, thinning, spacing), underplanting, and, underburning. For example, partial cutting regimes could retain a lodgepole and/or ponderosa pine component within an existing stand, thus encouraging future establishment of these species in the stand.

Stand structure manipulation and stand improvement

Manipulation of stand structure is an important tactic in the management of western spruce budworm. High stand densities increase stand hazard by providing an abundance of new foliage for larvae. Stocking and density control facilitates rapid growth of individual trees and permits selection of genetically superior trees that are more tolerant of defoliating insects. Density control can be achieved through harvesting and silviculture treatments such as thinning, spacing and underburning. Greater inter-tree distances achieved through density control promote full crowns and more vigorous growth so that tolerance to budworm feeding is enhanced. Attempts should be made to minimize the situation of large, full-canopy trees overtopping smaller, less well foliated trees. Overstorey trees provide a source of budworm dispersing down onto trees growing under the canopy radius.

Stands with chronic budworm defoliation should be stratified into one of the following four stand structures:

(a) high density, multi-layered, with scattered canopy gaps
(b) clumped, low density stands (often as a result of selective harvesting or thinning)
(c) variable density, multi-layered, patchy distribution with canopy gaps (often old selectively harvested stands)
(d) even-aged or single-storied, mature Douglas-fir stands.

In stand a, retaining the multi-layered or uneven-age structure is often the most desirable and practical option. Therefore, during harvest or thinning operations, each layer should be spaced to minimize overtopping of understorey trees and to maximize inter-tree growing space. Maximizing inter-tree spacing is also recommended for b. In c, small patch cuts (about 2 tree-lengths in diameter), with follow-up spacing of the residual stand, is recommended. If the existing stand structure and spatial arrangement of stems allow, overstorey and inter-mediate level trees throughout the stand should be removed to create small, even-layered patches of trees within the stand. This harvest and thinning procedure will result in small, even-canopy islands retaining or enhancing canopy gaps which are natural to these forests. In ecosystems where even-aged stands of Douglas-fir occur (d), even-aged management systems (e.g., shelterwood, seed tree) are the recommended silviculture system. Adequate inter-tree growing space is still essential to promote a healthy stand.

Fertilization of stands can be done to promote full crowns and increased tree growth. The relationship of forest fertilization to budworm defoliation is not fully understood at this time.

Short-term strategies: biological and chemical control

Foliage protection and population reduction are short-term direct control strategies that utilize chemical or biological insecticides. Direct control should be considered when moderate to severe defoliation is predicted in a stand for the following year. Several biological and chemical insecticides are presently registered for use against the western spruce budworm. Timing of insecticide treatment should coincide with peak 4th or 5th instar depending upon treatment objectives.

The use of biological insecticides has proven to be very successful as a management option for the western spruce budworm. Environmental impact is minimal and efficacy is high, provided the insecticide is applied in a correct and timely fashion.

The most commonly used biological control agent for western spruce budworm is Bacillus thuringiensis var. kurstaki (B.t.k.). The efficacy of B.t.k. treatments is variable, depending on the following factors:

Application techniques for B.t.k.

Weather and timing of B.t.k. application must be carefully considered before spraying. Applications are done when the following parameters are met:

The best time to apply B.t.k. is when larvae are actively feeding. Budworm larvae tend to feed more during warm weather; therefore it is advisable to spray at these times. There should be no threat of heavy, continuous rain for at least 24 hours after spraying. Heavy rains could wash off spray deposit, and similarly, if foliage is wet at the time of application, spray droplets may not adhere to the foliage. Light rain, for short durations should not seriously impact the efficacy of treatment. As long as foliage is not wet to the point of run-off, some moisture in the stand is desirable as it will maintain a higher humidity. Table 16 lists suggested timing and dosage rates.

Both aerial and ground application techniques are available for applying B.t.k. However, aerial application gives the best coverage, and is generally the most economical method for large, continuous areas of mature forests. Either fixed or rotary wing aircraft may be used in the application of B.t.k. Aircraft should be equipped with spray systems capable of delivering droplets in the range of 80–120 median micron diameter at a constant pressure over an even, unbroken swath.

If aerial application of insecticides is planned, rotary wing surveys may be necessary to define spray block boundaries. Spray blocks should be mapped on 1:15 000 to 1:50 000 forest cover maps and color air photographs (creating air photo-mosaics of spray area). Spray blocks should encompass all high priority areas ensuring that the highest insect density areas are covered. Re-invasion by insects is reduced and priority timber types are thus protected by the spray. Spray block boundaries should be delineated using distinctive features such as:

Block boundaries must be well defined from the air by the features listed above or by other visual means. Boundaries can be marked using large, helium-filled balloons (white and yellow are the most visible), or by securing flags in tree tops.

Efficacy of B.t.k. treatment can be assessed by establishing sample lines inside and outside of treatment areas. Larval sampling procedures and calculation of mortality are outlined on page 43. Untreated check areas should have the same budworm population, cover the same elevational range, and have similar stand structure and composition as the corresponding treatment area.

Establishing sample lines for pre- and post-spray sampling:

  1. Sample lines should be at right angles to the predicted spray swath, and cover the range of elevations within the treatment area.

  2. Sample trees on sample lines should be a minimum of 50 m inside treatment boundaries and spaced about 50 m apart.

  3. At least 5–10 trees per sample line should be selected. The number of sample trees established is dependent on the size of area being treated (Table 17).

  4. Draw the location of sample lines (and number of sample trees) on a 1:15 000–1:50 000 scale forest cover map.

Population reduction versus foliage protection

Direct control strategies include population reduction and foliage protection. Foliage protection is the strategy most commonly adopted for budworm outbreaks in B.C. The aim is to reduce feeding damage in order to maintain tree vigor. Low levels of defoliation are acceptable (Table 7).

Population reduction can be applied to areas where little or no defoliation can be tolerated. This strategy is applied in the early stages of an outbreak, to reduce extremely high populations, thereby minimizing significant resource impacts.

Variation in the timing of direct control can achieve either foliage protection or population reduction. Higher insect mortality is achieved when later instars are targeted (5th or 6th), however more B.t.k. must be consumed per insect to achieve desired results, and more damage is incurred prior to treatment. Late instars are more open-feeding, consume greater quantities of foliage, and are thus more likely to encounter and consume a lethal dose of B.t.k.

Typically, peak 4th instars are targeted to minimize defoliation. Timing to achieve good foliage protection is difficult due to differences in host and insect phenology. Larvae remain feeding in buds until 4th instar, and are thus well protected from predators and the effects of a spray program. Larvae begin open feeding on the flushed shoots at about the 4th instar. Buds on overstorey trees should be >80% flushed prior to treatment and understorey trees should be close to 100% flushed.

Higher potency B.t.k. formulations, increased dosage rates, or double application of lower potency formulations, may be considered in the following situations:

Defoliation history of trees and stands, combined with the predicted level of defoliation, influence which tactic will be implemented. Stands that have already sustained significant damage over 2 or more years (moderate to severe whole tree defoliation) should be managed under the foliage protection strategy. This will decrease further significant damage from occurring. If the population reduction strategy is applied in this situation, considerable damage could occur prior to the treatment being applied. Figure 13 provides guidance in selecting an appropriate strategy. Table 18 and Figures 14 through 17 describe the decision process for determining when and where to apply direct control measures.

Figure 14. Decision-making process in multi-layered stands for doing predictive sampling surveys.
Figure 15. Decision-making process for treatments in a multi-layered stand with only current year defoliation.
Figure 16. Decision-making process for treatments in a multi-layered stand with 2 years defoliation.
Figure 17. Decision-making process for treatments in a multi-layered stand with >2 years defoliation.


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