Defoliator Management Guidebook Table of Contents]

Western hemlock looper

The preferred host of the looper is western hemlock, although during outbreaks, the looper feeds on almost any foliage, including broad leaved forest trees and shrubs. Although most outbreaks have occurred in mature and overmature hemlock and hemlock-cedar stands, some infestations have occurred in vigorous hemlock stands 80100 years old. Stand parameters influencing hazard are listed in Table 19.

Population dynamics and detection

The western hemlock looper is periodically destructive in coastal and interior forests, reaching outbreak proportions every 11 and 20-plus years, respectively. Outbreaks of the hemlock looper usually last about 3 years, after which they are generally brought under control by the action of parasites, predators, and diseases. Heavy rains during the moth flight period can reduce egg-laying and hasten the decline of an outbreak.

Larvae hatch from eggs in the spring. Feeding by early instars during May, June, and early July is light, and not particularly noticeable. As larvae grow larger, from the middle of July to October, they feed voraciously on both new and old foliage. The larvae are wasteful feeders, chewing off needles at their bases and thus causing the stand to appear yellowish-red and then brown in color. In heavy infestations, trees may be stripped in a single season. Defoliation starts in the upper crown, but as feeding progresses more and more of the crown is affected, increasing the risk of mortality. Late in summer, larvae are very mobile, crawling over tree trunks and shrubs, and dropping by silken threads from the trees to the ground. By fall, the ground may be littered with parts of needles, insect frass, and later by thousands of dead moths.

Predictive sampling techniques

Egg sampling is done in the fall when defoliation has been noticed, or when an outbreak is anticipated, to predict levels of defoliation the following summer.

  1. Samples of lichen found on the bole and branches of hemlock are taken from the lower crown of 510 trees per site, using pole pruners.

  2. Eggs can be counted using a magnifying lamp or by using a hot water extraction method to determine the average number of healthy eggs per 100 grams of dry weight lichen. The procedure is as follows:

  3. Healthy eggs must be distinguished from parasitized, infertile or old eggs as follows:

Predicted defoliation using the hot water method:

Long-term strategies

Well-spaced, even-aged, thrifty stands should be less susceptible and suffer fewer impacts from western hemlock looper defoliation. Promoting mixed species stands composed of less than 50% western hemlock, avoiding cedar-hemlock mixes, and preferring non-host species (Table 19) will also lessen susceptibility. Stand tending treatments such as spacing and fertilization will help maintain a healthy stand that is likely more resilient to western hemlock looper defoliation.

Short-term strategies

The recommended short-term strategy for managing western hemlock looper is to identify high hazard stands containing the highest historical frequency of infestations and monitor in years preceding an anticipated outbreak period. In eastern Canada, B.t.k. has been sprayed on outbreaks of eastern hemlock looper to control outbreaks. In British Columbia two formulations of B.t.k. were tested in 1993 and shown to effectively control the western hemlock looper. However, incorporation of the western hemlock looper into the registration of these two B.t.k. formulations is still pending. Therefore, at this time there are no biological insecticides regisitered for use against the western hemlock looper. There are some chemical insecticides registered for use in B.C.

Eastern spruce budworm

Population dynamics

The eastern spruce budworm is the most destructive defoliator of spruce-balsam forests in North America. Although mature balsam fir (Abies balsamea) stands are most susceptible, outbreaks are common in northern B.C. and the Yukon in white spruce, Picea glauca, stands containing little or no Abies. Outbreaks usually cover extensive areas and last several years. Short-term population fluctuations often occur during outbreaks. Such fluctuations influence the rate of tree mortality thereby affecting control programs and potential salvage operations. Unfortunately, these fluctuations usually can not be reliably predicted.

There is considerable evidence that seasonal warm and dry conditions which prevail in the boreal white and black spruce (BWBS) zone in B.C. and the Yukon favour eastern spruce budworm survival (Table 20). Fire suppression and the retention of overmature spruce stands render forests more susceptible to attack. Older, more open-grown stands have more favourable eco-climate compared to the cooler, more humid climates of dense, young stands. Spectacular increases in outbreaks have also coincided with unusually heavy production of staminate flowers on spruces and balsam. Impacts of repeated defoliation are similar to those caused by other forest defoliators and are most like the damage caused by the western spruce budworm.

Dispersal occurs during all the active stages of the budworm. During emergence from the eggs and hibernacula, windborne dispersal of small larvae is common and can result in the extension of outbreaks. The distance small larvae can be transported by air currents is not well known but is probably limited to a few kilometres. Moth dispersal, particularly along prevailing winds, is likely the most significant dispersal mechanism. The rapidity with which the size of outbreaks has increased would indicate that, at least at certain times, population increases occur over large areas rather than as a result of spread from discrete centres.

Natural mortality factors that limit the size, intensity, and duration of outbreaks include predators, parasites, pathogens, starvation, and weather effects. Overwintering mortality seldom exceeds 15%, but prolonged freezing temperatures and/or rain during and shortly after emergence in the spring may sometimes have a major impact on outbreak development. Approximately 6070% mortality of small larvae can also occur during fall and spring windborne dispersal.

Population monitoring and predictive sampling techniques

Annual aerial sketchmapping should be done in all high hazard ecosystems (Table 20) in late June when defoliation is most visible. Interpretation of the per cent current defoliation is provided in Table 21 and is the first step in calculating individual stand hazard rating.

Stand level surveys can then assign further hazard rating factors based on the per cent old defoliation, top-kill, tree mortality (Table 22), and vigor (Table 23).

Prior to the preparation of silviculture prescriptions in high hazard ecosystems, both population monitoring and predictive sampling should be done. Results of such sampling should be considered and incorporated into recommendations in the final prescription.

Egg mass sampling surveys

Eastern spruce budworm egg mass sampling should be conducted in late summer or fall (mid-August through late October). The procedure is similar to that described for the western spruce budworm, but since mature white spruce exceeds 30 m in height, branch samples are usually obtained by felling at least one tree per stand and taking six 45-cm branch samples: 2 from each of the upper, mid- and lower crown. Table 24 interprets egg mass densities and forecasts budworm populations and estimates expected defoliation for the following year.

Egg mass surveys tend to underestimate increasing budworm populations and overestimate populations during infestation collapses, but are reasonably accurate for forecasting epidemic populations.

L2 surveys

If egg mass sampling was not done or severe winter temperatures have occurred, it may be appropriate to conduct L2 surveys in early spring prior to larval activity and any planned control program. Larvae are "force reared" which breaks their diapause so that an early confirmation of their population densities can be made. The methodology is similar to that used for western spruce budworm except for the constraint of collecting limited branch samples from 30 m high mature spruce. Once this information has been compiled, the sum of the rating factors from the final columns of Tables 2124 can now be inserted into Table 25 to calculate the total rating for eastern spruce budworm.

If the overall hazard rating from Table 25 is moderate or higher (suggesting stand treatment) refer to Figure 18.

Budmining surveys

Budmining surveys in the spring are usually only undertaken if L2 or egg mass sampling has not occurred or inclement weather has intervened since the initial survey. Usually such effort is only expended in potential spray blocks to confirm previously forecasted population levels. The methodology is the same as for western spruce budworm with the exception of obtaining a smaller number of samples per stand from tall spruce.

Larval sampling

The purpose and methodology of larval sampling is similar to those used for western spruce budworm with the following exceptions. Larval densities are usually based on the number of new shoots per branch which often provides a narrower variance during statistical analysis. Given the presence of obstructing ground vegetation during the collection period, branch samples are labelled, bagged, and brought to a central processing facility where larvae are extracted in "beating drums."

Long-term strategies

Alternate species and species mixes

The river valleys in the BWBS zone in northeastern B.C. and the Yukon are characterized by climax stands of even-aged white spruce which are highly susceptible to eastern spruce budworm attack. Attempts to change stand composition to less susceptible, and less valuable, species must be drastic if they are to overcome the natural ecologic succession in such stands where spruce eventually tends to follow spruce. Difficulty arises because of the moderate tolerance of spruce to growth conditions, and any form of partial cutting in wetter high hazard zones favors the regeneration of spruce at least as much as that of any less susceptible conifer species.

Even-aged mosaics

The long-term strategy most likely to be effective in reducing susceptibility and vulnerability to eastern spruce budworm is the creation of even-aged mosaics of differing age classes. This concept is based on the observation that susceptible stands isolated by non-susceptible areas do not sustain as much damage as similar stands located within susceptible forests. This is because in remote stands more drifting larvae are lost by dispersion than are replaced by in-drift. This suggests establishment of a sort of checkerboard of even-aged patches in which the oldest most susceptible forest is isolated as comparatively isolated patches among a younger less susceptible forest.

Uneven-aged silvicultural systems

There are few budworm-susceptible spruce sites within the BWBS zone where uneven-aged silvicultural systems are recommended because of the high risk of windthrow to the residual stand. Other practical constraints include logging damage to the residual stand and possible enhancement of tomentosus root disease. Such harvesting creates openings that maintain continuous, deep-crowned, mature or nearly mature canopies with abundant "sun foliage" and flowering. Once infested, the overstorey serves as a reservoir of larvae that disperse downward, exposing young age classes to damage.

Short-term strategies: biological and chemical control

As with western spruce budworm, foliage protection and population reduction are recommended short-term strategies for the eastern spruce budworm. Direct control with registered biological or chemical insecticides should be considered when the criteria from Table 6 have been met and other long-term silvicultural options by themselves are inadequate. There are many small watercourses, wetlands, and riparian zones in the BWBS mk2 that would severely restrict the use of a chemical insecticide, therefore the use of the biological insecticide B.t.k. is preferred. The efficacy of B.t.k. against eastern spruce budworm is variable and depends upon the same factors listed for western spruce budworm.

The application techniques and strategies for using B.t.k. against eastern spruce budworm are similar to those employed against western spruce budworm. Given the northern latitude of the BWBS biogeoclimatic zone, on clear mornings aircraft can become airborne as early as 0300 hours and spray from approximately 03300800 hours.

Western blackheaded budworm

The western blackheaded budworm is an important defoliator of western hemlock. Western hemlock and true firs are the preferred host species, but spruce and Douglas-fir can also be fed upon (Table 1). The most serious defoliation occurs on the Queen Charlotte Islands and northern Vancouver Island. Outbreaks in the interior are usually not severe enough or last long enough to cause significant damage. In general, high budworm populations appear to develop and build in mature stands and subsequently spread to immature stands. Stand parameters influencing hazard are listed in Table 26.

Population dynamics and impact

On the coast, outbreaks generally last two to three years before declining to low levels. In the interior wet belt, they generally last only a year or two. Outbreaks occur roughly every 10 to 15 years. On the coast, outbreaks of blackheaded budworm are often associated with outbreaks of hemlock sawfly, and in the interior, they seem to follow outbreaks of western hemlock looper.

Healthy western hemlock appear to be able to withstand one year of severe defoliation without sustaining serious damage, often recovering within a year or two with minimal amounts of top-kill or growth loss. In the Queen Charlottes and Vancouver Island, outbreaks can last up to three or four years, thereby impacting tree health more significantly. Impacts from blackheaded budworm defoliation start to appear after two years of moderate to severe defoliation. Defoliation impacts assume three main forms: incremental growth losses, top-kill and mortality. Radial and height increment are reduced by 50% for approximately four years following collapse of the outbreak. Top-kill results in the loss of previous height increments and can cause deformities in the stem (forks and crooks) as compensating laterals assume dominance. Top-kill is particularly a problem in immature stands. Stagnation of height growth in mature trees is another impact of defoliation. Although this budworm is not known as a tree killer, mortality losses can occur when trees receive multiple years of moderate to severe defoliation.

Detection, population monitoring, and predictive sampling

Defoliation appears as a reddish haze in summer; faint initially (first year) but becoming very bright during the summer (end of July) at the peak of the outbreak. Hemlock sawfly populations often reach outbreak levels at the same time as blackheaded budworm, therefore defoliation can sometimes be confused between the two insects. Walkthroughs of the affected area should be done to confirm the causal agent.

Larvae and egg sampling can detect building population years prior to outbreak. From egg sampling results during the early fall, defoliation predictions can be made for the following summer.

Larval sampling: To dislodge blackheaded budworm larvae from sample trees, beat branches with a 2.5 m pole held over a 2 x 3 m ground sheet. Each sample plot consists of three trees. Annual sampling in this manner will detect increases in budworm populations prior to the occurrence of outbreaks. For determining efficacy of direct control programs, see larval sampling procedure.

Egg sampling: Sample a 45 cm branch tip per tree from each of the north and south aspects, from the mid-crown of 10 trees per sample location. The average number of healthy eggs per branch provides an estimate of next year's defoliation levels as listed in Table 27. Visual assessment or the hot water method, as outlined for western hemlock looper egg sampling, should be used to count the number of eggs per sample.

Long-term strategies

Stand tending treatments such as spacing and fertilization will help maintain a healthy stand that is likely more resilient to budworm defoliation. Well spaced, even-aged mature stands should be less susceptible and suffer fewer impacts from budworm defoliation.

Short-term strategies

The recommended short-term strategy for managing western blackheaded budworm is to identify high hazard stands containing the highest budworm population levels, and treat with B.t.k. Damage thresholds will be reached when an immature stand (less than 60 years of age) has received one year of moderate to severe defoliation and a second year of moderate to severe defoliation is predicted (i.e., two years of moderate to severe defoliation is unacceptable). Refer to page 49 for details of planning and implementing a B.t.k. spray program.

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