Environment

Urban Forestry

Western Spruce Budworm Control Strategies

Population Monitoring
The first step in a control program is to determine whether the WSB is currently causing visible damage and whether serious defoliation (moderate to severe) is predicted for the following spring. City employees and the general public visit the susceptible forested lands of interest frequently (in particular the parks) so current damage can be easily noted.

Defoliation is most visible in the late spring, when damaged needles are most vivid in colour (a reddish brown) before wind and/or rain knock them off the trees. If defoliation is noted (as it was in 2007) then the next step is predictive sampling in the form of egg mass surveys (BCFS 1995). The survey is conducted in the fall after it is certain that egg laying is complete.

Figure 4 - Western spruce budworm, new egg mass (top) vs. old egg mass (bottom)

Branch samples 45 cm long are taken from mid to upper crown, north and south site, on ten dominant to co-dominant trees per site. Chosen sample trees are at least 30 meters apart.

These samples are then examined with a magnifying lamp to find all new and old egg masses (Figure 4), as well as any signs of parasites or predators.

The average width times the length of each branch is recorded, and the number of new egg masses per square meter is calculated for the site. This number is then translated into a prediction of expected defoliation for the following spring. More than 50 and 150 egg masses per 10m2 of foliage indicate a potential for medium and severe defoliation, respectively.

Direct Control
If fall egg mass sampling indicates that defoliation will be moderate to severe the following spring, direct control measures can be undertaken by spraying infested stands with Bacillus thuringiensis var. kurstaki (Btk). Btk is a bacterium that is found naturally in the soil and is known to affect only lepidopteron larvae that consume it. It has no known toxic effects on humans or other mammals, plants, birds, fish, frogs, honeybees or other beneficial insects. It is registered for use in organic food farming, and breaks down in about three to seven days (Valent BioSciences).

Btk must be applied in a specific fashion at a specific time in the WSB life cycle to result in significant population reductions. It is important to note that the results of treatment with Btk do not result in budworm population eradication. The objective is to reduce the population to natural, endemic levels where trees are not permanently damaged by minor defoliation.

When deciding on priority treatment areas for forested public lands in BC, various criteria are considered:

whether defoliation has been present for a year and if moderate to high defoliation is predicted for next year
whether populations are increasing in intensity and expanding in range in the general area
if stands are of high working forest value (e.g. have silviculture investments such as thinning and fertilizing or are in a woodlot)
whether stands have a moderate to high density understory
are wildlife habitats at risk
is it a high value recreational/aesthetic area such as a cross-country ski/hiking zone
are the biogeoclimatic subzones considered high hazard for damage

Btk has been used on forested stands when required in the Kamloops Forest District on an operational basis since 1991, after three years of research project use from 1987 to 1989. In response to the large jump in defoliation damage recorded aerially in 2006, 17,925 ha were treated in the spring of 2007. This was the first treatment program undertaken in this district for the past six years, although other areas in the Southern Interior Forest Region were sprayed during this period.

Budmining surveys are required in the spring within proposed treatment areas to confirm that budworm populations have not declined significantly during the winter months. This survey is conducted just as the majority of the overstory tree buds start to swell. Trees chosen for sampling are dominant to co-dominant representative Douglas-fir trees.

At each budmining site, five to ten 3-tree clusters are chosen at random, at least 40m apart. From each Douglas-fir tree, two 45cm branch tips are collected from mid crown and the percentage of mined buds vs. uninfested buds are tallied.

Figure 5 - Collecting budmining samples

If 15 - 30% of the buds are infested, defoliation is expected to be moderate and over that level defoliation is predicted to be severe.

If population levels are still high and treatment is confirmed, bud development as well as larval activity and growth need to be carefully monitored in the weeks leading up to the spray, both for synchrony and spray timing. If tree and larval development are not in synchrony, significant larvae mortality can still occur. Correct spray timing is very important. If Btk is applied too early when the budworm are still feeding inside the buds it will not be effective.

Conversely, if it is applied very long after bud flush treatment may result in increased larval mortality but foliage damage that year will be high. The best treatment timing is when the larvae are primarily 4th instar and buds on the overstory trees are at least 80% fully flushed (BCFS 1995).

Once an area is confirmed for treatment, a global positioning system (GPS) file will need to be created in a format that can be loaded into the spray aircraft satellite location system. The aircraft system results in very accurate treatment of the target area, and assures non-target areas are not treated.

Leading up to treatment the general public should be made aware of the impending treatment, at a minimum through advertising and preferably with an article in the local newspapers. The public tends to be much more supportive of a control program when they understand how safe Btk is.

The Btk formulation has some important limitations that must be considered during treatment. It is applied in a very fine droplet form, which evaporates at low humidity and tends to drift with high winds. A slight wind is desirable to drive the spray down into the forest canopy, but winds greater than 15 km/hr or gusty conditions are not acceptable. Additionally, a heavy rain within a few hours of application greatly reduces the effectiveness of the spray, as it washes the Btk off the foliage.

Research has shown that evaporation of Btk before it reaches the ground is determined by both temperature and humidity. The following graph (Figure 6) has been utilized since 1998 to determine optimum spraying conditions based on this relationship (Valent BioSciences).

Figure 6 - Temperature/Relative Humidity relationship depicting safe and unsafe meteorological conditions for spraying undiluted aqueous formulations and diluted formulations of Btk (Valent BioSciences).

Very site-specific weather data is required to conduct spray operations successfully. The data is obtained by setting up mobile weather stations at each treatment site. Each station consists of wind, temperature and relative humidity monitoring devices. Stations are required to be manned during the spray for a period beginning 30 minutes before and ending 30 minutes after the spray. Weather readings are recorded every 15 minutes. The operator also keeps track of possible storm build-up and monitors the drying of foliage if it had rained previous to treatment. Relatively dry foliage is required for the Btk formulation to stick properly.

Treatment of any or all of City of Kamloops forested land could easily be completed in one morning, weather permitting (Figure 7). Weather forecasts from the day before would assist in making the tentative decision to spray, but on ground weather reports in the early morning are the deciding factor.

Egg mass sampling is recommended for the fall following treatment, to determine if population reductions have been achieved. It is a rough measure of the success of Btk treatment, as natural population decline factors such as parasites and predators cannot be accounted for. In the early days of MOFR treatment programs, intensive larval sampling was conducted in spray areas and comparable non-treatment control areas, to account for natural mortality vs. Btk application mortality. Results almost always showed that Btk accounted for a large portion of the budworm mortality. Larval sampling was a costly endeavour, so it was discontinued when it consistently showed treatment worked. Egg mass surveys were employed instead to predict future defoliation.

Figure 7 - Aerial Btk treatment with a Hiller UH12ET helicopter

Stand Structure Manipulation for Hazard Reduction

In the dry, hot ecosystems of natural forested land in the City of Kamloops, Douglas-fir generally requires a multi-layered structure for stand regeneration. High density multi-layered stands have the highest hazard in regards to western spruce budworm damage. These stands provide an abundance of new foliage (preferred food source) for the budworm, and provide an abundant target area for larval interception when they disperse. Understory trees are particularly susceptible.

Tree vigor is also a factor: wider spaced trees are healthier due to increased availability of moisture and sunlight, which in turn produces larger, more vigorous crowns. These trees have a higher tolerance to budworm feeding. Additionally, the wider spacing interferes with successful budworm dispersal. Larvae that either fall to the ground or land on non-host trees are likely to succumb to starvation, exposure or ground-dwelling predators.

To reduce the spruce bud worm and wildfire hazard in high density multi-layered Doulas-fir stands, thinning is being conducted in all of the wildfire fuel management areas. Stands are being spaced to minimize overtopping of understory trees and to maximize inter-tree growing space. When possible, the most suppressed and/or most defoliated trees are selected for removal.

Stands with a mix of host and non-host tree species also have a lower hazard rating for budworm. Mixed tree species reduce stand suitability for sustaining high budworm populations and hence reduce stand damage. In the City of Kamloops interest areas, mixed Ponderosa pine and Douglas-fir forests are encouraged where possible. In general, mixed species promote healthy forests.

For More Information: http://www.for.gov.bc.ca/hfp/publications/00198/wsbw.htm

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Urban Forest
955 Concordia Way
Kamloops, BC V2C 6V3
ph (250)828-3461
fax (250) 828-3790
email scook@kamloops.ca

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