BCWEEVIL: A simulation model of the joint population dynamics between spruce weevil and Sitka spruce, over the lifetime of a plantation

The spruce terminal weevil Pissodes strobi (Peck) is a major pest in western spruces, attacking trees of all ages beyond the juvenile stage, killing the leader and causing tree distortion. This paper describes a computer simulation model of the joint population dynamics within this tree/pest system, over the lifetime of a plantation, and is combined with the Province of British Columbia's Tree and Stand Simulator (TASS) to drive individual tree growth and stand establishment and development. The model differs from current models of this system, and of other similar systems, in two important ways: (1) it simulates the entire life of a stand, from planting until harvest, and (2) it is based on the underlying biological processes that govern behavior of individual weevils on and in each tree.Each model simulation begins by planting a stand using tree materials from seed or clonal hedge orchards, choosing trees either individually and independently or in groups represented as clones or families. Stand growth and mortality are advanced through a juvenile period, after which weevils appear in the stand. From this time until harvest, the model simulates daily changes in the weevil populations on each tree, tracking mortality, oviposition, and juvenile maturation and emergence, as well as weevil movement from tree to tree. Once a year, the code projects tree mortality and growth, taking into account within-stand competition and damage to leaders caused by weevil attacks. At harvest, the model computes an estimate of the merchantable timber produced by the stand.As illustrations of model output, we present (1) simulated average numbers of adult and juvenile weevils in stands generated using materials from seed orchards, both throughout a single season and also through the years from stand planting until harvest; and (2) the differential effect of weevil damage on two tree genotypes, one resistant and one susceptible, in a stand composed of adjacent clonal blocks. The results of model simulations are in agreement with some of the population dynamics statistics observed in plantations, suggesting that the model reflects biological realism and can be used as a research or management tool.