A Comment on “Feeding and Oviposition Preferences of White Pine Weevil (Coleoptera: Curculionidae) on Resistant and Susceptible Sitka Spruce Clones in Laboratory Bioassays”

Abstract

Associations between the density and distribution of cortical resin canals of host trees and their resistance against insects have been the subject of many studies (Stroh and Gerhold 1965; Wilkinson 1983; Tomlin and Borden 1994a, 1994b). None, however, has demonstrated a causal relationship between number, density, or distribution of resin canals and resistance. For demonstrating causal relationships between resistance and its causative factors and identifying distinct mechanisms of resistance, our understanding must advance beyond associations. The premise of Alfaro (1996) constitutes an initial step for exploring a possible causal relationship between the density of cortical resin canals and resistance by proposing and testing a biologically meaningful hypothesis. He examined feeding and partial oviposition patterns on resistant and susceptible young grafts of Picea sitchensis (Bong) Carr. by Pissodes strobi Peck. in a greenhouse experiment and feeding patterns on the terminal and penultimate internodes of lateral branches in petri dishes. The main finding was that feeding and oviposition are several times greater on the penultimate internode of the main stem than on the leader. The author then hypothesized that this pattern could be the result of a higher number of resin canals, a higher proportion of the area being occupied by resin canals, or a higher density of the constitutive cortical resin canals in the leader. In a separate sample of three resistant and three susceptible trees he found that the density of resin canals and the area occupied by them per unit area of the are higher in the leaders than in the corresponding penultimate internodes. From these data he concluded, bark resin canals play an important role in the resistance of spruce to weevils not only within his experiment but also by extrapolation of this inference to explain how resistance may function in the field. …