Sitka Spruce Weevil Hazard Decision Tool for Vancouver Island
Silviculture guidance for Sitka spruce on Vancouver Island
- Plant seedlings sourced from Sitka spruce seed with the highest
resistance rating available for your planting area – check the
Seed Planning and Registry system (SPAR) for availability.
- Consider higher planting densities. Leave at least 1250 sph by age 10-12
– aim for 1400-1600 if possible. Thin later, or avoid thinning until trees
grow taller than 15 m when weevil hazard decreases. Higher densities of any
species shade the site, although there will be a trade-off between growth
and wood quality. Lower density = better growth, but higher weevil attack.
- Proportion or percentage of Sitka spruce had no effect, so pure and
mixed stands will have similar levels of weevil impact.
- Maintain deciduous or other overstorey – alder has many benefits. It
enables good height growth before and after the canopy leafs out,
substantially reduces weevil incidence by shading the leaders during the
growing season, and provides nitrogen inputs to soil. Other conifers will
also somewhat reduce weevil attack, but may compete for resources.
- In areas with high to extreme weevil hazard, consider planting alternate
species or limiting Sitka spruce to <30% of the stand, except where
biodiversity is the primary objective. Where trees are not planned for
harvest, maintain Sitka spruce on the landscape. Weevil attacks will not
affect the value as merchantability is not an issue.
- To increase wood quality in the mature stand, clip multiple leaders –
this can be done most cost-effectively during stand surveys.
- Avoid fertilizing unless trees are over 12 m tall. More succulent,
larger leaders increase attack severity, but in taller trees the likelihood
of attack decreases. For interior spruce, studies show fertilization may be
- Most silvicultural activities are unlikely to cost-effectively reduce
- Sitka spruce density has little effect on attack rates in moderate
to high hazard sites.
- Thinning increases attack rates, at least until trees grow above
weevil attack limits, around 15 m.
- Leader clipping is extremely costly and must continue over multiple
years to work.
- Site preparation is unlikely to have an effect since the window for
weevil damage lasts for over a decade.
- Brushing tends to increase attack rates by exposing regeneration to
- Mounding to create plantable spots in imperfectly drained sites is
the one exception that could lower attack rates.
- Suitable sites for planting Sitka spruce for timber objectives are
typically rich to very rich in subhygric to subhydric sites, and medium to
high benches. For non-timber objectives, Sitka spruce is also suitable in
riparian sites, sites affected by ocean spray, and some well- to moderately
well-drained mesic and richer higher-elevation sites up to 800 m with low
Updated May 2009