White Pine Weevil
White pine weevil or spruce leader weevil (Pissodes strobi) is a weevil in the family Curculionidae (weevils or snout beetles) in the order Coleoptera. Adult weevils are brown with white spots; they occur in spruce and pine ecosystems throughout Canada.
Adult weevils make short flights early in the spring to disperse after overwintering. Adults lay their eggs at the tip of the previous year’s leader in May and early June. The eggs are laid inside excavations in the bark and are held in place with fecal matter. The larvae hatch in approximately ten days and feed downwards along the leader. The larvae consume the cortex and phloem killing the previous year’s leader and causing the current year’s extending leader to die and curl into the characteristic “shepherd’s crook” form. The larvae pupate in chip cocoons at the base of the leader and new adults emerge in August or September of the same year. If weather conditions are unfavorable, then larvae or pupae may overwinter and emerge in the following spring. The life cycle is completed within one year.
White pine weevil attacks several commercially important conifer hosts. The most important hosts in British Columbia are sitka spruce (Picea sitchensis), Engelmann spruce (Picea engelmannii) and white spruce (Picea glauca). Other hosts in North America include eastern white pine (Pinus strobus), Jack pine (Pinus banksiana), and Norway spruce (Picea abies).
Stem deformation occurs when lateral branches below the attacked leader compete for apical dominance. This can cause a forked or dwarfed tree form. Repeated attacks on new dominant laterals cause severe stem deformation over time. Thus, weevil attack causes growth losses, a reduction in wood quality, a decrease in lumber recovery, and potentially tree death.
Once a suitable host tree is identified, successful attack depends both on weevil numbers and host suitability. Weevil larvae must be present in sufficient numbers overcome the host tree’s defense effort. Alfaro et al. (1996) found that adult emergence was very low in trees with fewer than 60 egg punctures per leader.
Currently, the efforts to control weevil attack have focused on risk assessment and reduction. Weevil outbreaks are facilitated by monocultures of host trees on warm, highly productive sites. Management recommendations for controlling weevil outbreak include dense replanting of host trees, planting non-host species, and planting a species mixture that will provide deciduous overstory (shading) for the developing host trees. Although it is unlikely that these methods will completely eliminate the weevil from commercially important plantations, the impact of weevil damage may be reduced. Tree breeding programs initiated by the British Columbia Ministry of Forests, Lands, Natural Resource Operations, and Rural Development have thus focused on the identification of resistance traits in Sitka spruce. Fill-planting (supplemental re-planting of areas with patches of dead or dying trees) resistant genotypes in stands of susceptible trees can also reduce damage due to white pine weevil.
Alfaro RI. 1996. Role of genetic resistance in managing ecosystems susceptible to white pine weevil. For. Chron. 72(4):374-380.
Alfaro RI, Borden JH, Fraser RG, and Yanchuk A. 1995. The white pine weevil in British Columbia: Basis for an integrated pest management system. For. Chron. 71(1):66-73.