Influence of feeding by Zeiraphera canadensis (Lepidoptera: Tortricidae) on growth of white spruce: larval density - damage and damage - shoot production relationships.
Field studies were conducted in New Brunswick, Canada, to evaluate the relationship between larval density of Zeiraphera canadensis and damage to current year's shoots of white spruce, Picea glauca and the response of white spruce, in terms of shoot production, in the year following damage. The length of undamaged shoots was closely related to the length of buds, the length of the previous internode, and shoot basal diameter. Basal diameter explained 78-94% of the variability in shoot length in the absence of herbivory and was chosen to estimate potential growth of damaged shoots. The number of larvae per shoot, shoot size, whether shoots were broken at a feeding scar, the amount of previous herbivory, and the position of damage along shoots explained 24-87% of the variation in 6 damage indices (i.e. area and percentage of shoots defoliated, area and percentage of cortical tissue consumed, percentage shoot length reduction and incidence of shoot breakage) examined over 2 years and at 2 sites. More absolute (defoliation area, scar area), though less relative (percentage of shoot defoliated and scarred), herbivory occurred on larger than on smaller shoots with equivalent numbers of larvae. Similarly, larger shoots suffered greater relative reductions in length than smaller shoots with similar numbers of larvae but were less likely to break at a feeding scar. When fed upon by similar densities of larvae, trees which had previously experienced high intensities of herbivory suffered greater shoot length reduction and, to a lesser extent, relative defoliation and scarring than trees subjected to lower intensities of previous herbivory. Feeding near the active meristem of shoots caused greater reductions in shoot elongation than more proximal feeding by the same number of larvae. Consequently, through impacts to shoot elongation, herbivory to the distal portion of shoots resulted in greater relative defoliation and scarring than more proximal herbivory. The number of shoots produced in the year following damage was not directly related to larval densities but was positively associated with shoot breakage. Feeding by 4 or more larvae on leaders always resulted in production of multiple leaders the following growing season. Similarities in larval density - damage and damage - tree response relationships between years and sites suggest they are robust and widely applicable and could form the basis of the first economic injury level for a forest pest.