Relationships between Pikonema alaskensis larval density and shoot growth and production in young black spruce

Six years of field experiments were carried out to establish relationships between the density of larval yellowheaded spruce sawfly, Pikonema alaskensis (Roh.) (Hymenoptera: Tenthredinidae), and current-year shoot growth and production in juvenile, open-grown black spruce, Picea mariana [Mill.] B.S.P. In manipulative sleeve-cage experiments, larval density explained 36–65% of defoliation on branches and 27–37% of variation in shoot length in the year following defoliation. The negative impact of larval feeding on shoot elongation increased with each year of herbivory, resulting in a nearly 31% reduction in tree height after 5 years of severe (i.e., c. 70% or more) defoliation. Production of current-year shoots was not influenced until 2o years following the initial bout of larval feeding and generally declined each year thereafter. Although a small but variable number of dormant (i.e., epicormic) shoots were produced each year, there was no apparent association with defoliation intensity. In field surveys, egg, mid-instar, and late-instar larval density explained, respectively 8%, 18%, and 33% of variation in shoot length growth in the following year. Despite some branches and trees sustaining multiple years of severe defoliation, there were no instances of either terminal shoot mortality or top kill (i.e., upper stem and branch mortality). Our study provides essential insect density–defoliation and defoliation–damage relationships for P. alaskensis in black spruce that may aid in establishing the first economic injury level for this destructive insect pest in Atlantic Canada.

► We provide relationships between sawfly density and growth loss in black spruce.
► We used a combination of field survey and manipulative sleeve cage experiments.
► High larval densities reduced shoot growth and production in subsequent years.
► Young black spruce exhibited a relatively weak compensatory response to herbivory.
► Results provide a key component to the management of Pikonema alaskensis in spruce.