Sitka Spruce Weevil Hazard Decision Tool for Vancouver Island

Step 1 Step 2 Silviculture

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 effective.
  • Most silvicultural activities are unlikely to cost-effectively reduce weevil hazard.
    • 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 higher temperatures.
    • 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 snowpack (MHmm1).

Updated May 2009