Sitka Spruce Weevil Hazard Decision Tool for Vancouver Island

Step 1 Step 2 Silviculture

Additional Information

Example: your degree day sum is 890 = baseline hazard is moderate to high

Site 1 (reduced hazard compared to baseline)

Site 2 (increased hazard compared to baseline)

imperfectly drained

warm aspect

in the fog belt

CWHxm1 or CWHvm1

plant at 1600 sph

very rich




Site series CWHxm1/06-08 or CWHvm1/07

hazard will decrease

expect annual attack rates around 20%, moderate

hazard will increase

expect annual attack rates of at least 40%, high

Degree days
Weevils need to accumulate at least 888 degree days above 7.2C to grow and successfully reproduce in Sitka spruce leaders. Approximately 100 larvae feeding will kill an average leader – if only a few weevils attack, it may recover. Observations specific to Vancouver Island show a lower degree days threshold. Other site factors must be considered too, since they affect the actual leader temperature, which limits weevil growth and development.

Attack rate can be % of trees attacked, either annually, cumulatively, or another index. Attack rate is strongly correlated with attack severity. Attack severity categorizes the impact on a stand in terms of growth form and volume or height loss. Trees that have been attacked only once or a few times by the time they reach 15-20 m tall often recover and have straight boles with no visible defects. This is especially true for hypermaritime subzones and the cooler fog belt areas of northern Vancouver Island.

BEC - Biogeoclimatic ecosystem classification unit
BEC variants can quite variable climatically, even though sites within a variant are more similar than sites across variants. Most environmental variation within a BEC variant is captured at the site series level. Sitka spruce is generally suitable on moister, richer site series. Microsite conditions strongly affect weevil hazard as well by influencing the leader and the forest floor, where the weevil overwinters. The CWHvm1 had especially high environmental and climatic variability and also a wide range of attack rates.

Fog belt
Sites in the fog belt have lower attack rates because the cooling effect reduces the actual number of degree days compared to that predicted by the model.

Ss leader position
Leader temperature is a key factor, since it limits weevil development. Exposed leaders accumulate degree days faster than predicted by the model for the site – some studies show they are 5-10 degrees warmer than the ground surface. Shading reduces degree day accumulation. It also reduces leader vigour and growth, but Sitka spruce stands regenerating under a canopy (e.g., alder) have much less weevil attack, possibly for these reasons.

Retaining a deciduous (alder, cottonwood) overstorey has been shown to substantially reduce weevil attack rates, while allowing the Sitka spruce to put on height increment in the spring and fall while the canopy is relatively open. The deciduous overstorey can be managed as a shelterwood, and removed once the spruce grow taller than 12-15 m. Sitka spruce may suffer from competition under a coniferous overstorey.

SNR - Soil nutrient regime
Very rich sites have far higher attack rates than nutrient-poor or medium sites. Fertilized sites also have higher attack rates for this reason: thicker leaders can support more weevils and more offspring.

Mineral soil depth
Sites with deeper mineral soil had more weevil attack; depth corresponds roughly with site nutrition. Deeper, richer soils support more nutritious, thicker leaders.

Organic soil depth
Sites that have shallower litter (LFH) or organic horizons have slightly more attack. This depends on the substrate and drainage, but in richer sites with higher rates of nutrient cycling (e.g., sites with Moder to Mull humus) the litter layer thickness reflects a richer site. On the other hand, sites with deeper organic horizons (e.g., Om, Of, Oh) are poorly drained and have low attack rates.

Sites with imperfect drainage have lower attack rates than sites that are well- or moderately well-drained. This may be because the substrate is not suitable for weevils, or because of the lower vigour and smaller size of leaders. Check for gleying or mottling in soil horizons during the growing season, high water table during the fall and winter, sphagnum moss, or other plant indicators of seasonally fluctuating and high water tables.

Sitka spruce height
Sitka spruce trees between 2 and 18 m tall are most susceptible to attack, which peaks around 10-12 m. This age range corresponds roughly to 5-20 years. Leaders on younger trees are too small, and older trees are too tall for weevils to crawl up and reach the leader after overwintering in the forest floor duff.

Increasing stand density (including regeneration density) above 1250 sph reduces attack rates somewhat – consider keeping density at or above 1600 sph to maintain full stocking and reduce weevil hazard. This density can include spruce stems, total stems, regeneration, or total stand density.

CWD - Coarse woody debris
Sites with >40% coarse woody debris covering the ground had less attack. There are several possible explanations: these sites often had less developed forest floors, large abundant CWD may also physically obstruct soil weathering, or soil acidity may increase as CWD decays.

Warm aspect (135-285 ) sites had only a small increase in weevil attack rates. The effect depends on slope and exposure.

Indicator plants
Some indicator plant species showed consistent trends with attack, but be aware that species cover varies with geographic area and stand age and density.

Indicator plant species

Observed indicator value for weevil hazard

Increasing with higher site hazard


    Richer sites


    Richer sites, well-drained

invasive or weedy pioneer species:

grasses, western buttercup, Scotch broom

    Relatively recent disturbance

    Nutrient-medium sites

    Moderately-well drained

Oregon beaked moss

    Longer time since disturbance

    Moderately well to well drained sites



Decreasing with higher site hazard


    More acidic soils / Mor humus

    Poorer sites


    Imperfect drainage

lanky moss

    Nutrient-poor sites

    More acidic soils / Mor humus

oval-leaved blueberry

    Mor to Moder humus

    Poorer to nutrient-medium sites

    Moderately well to well drained sites


    Mor to Moder humus

    Poorer to nutrient-medium sites

    Less frequent with poor drainage


    Recent disturbance / exposed mineral soil

    More frequent with richer sites

    More frequent in well-drained sites


Updated May 2009