Sampling methods for evaluating yellowheaded spruce sawfly density and defoliation in juvenile black spruce stands

Field surveys were carried out in central Newfoundland to establish sampling procedures for evaluating the density of yellowheaded spruce sawfly, Pikonema alaskensis (Roh.) (Hymenoptera: Tenthredinidae), and associated defoliation in black spruce, Picea mariana (Mill.) BSP (Pinaceae), stands. Sampling defoliation only for shoots on the main branch axis and second-order branches in whorls 1, 2, and 4 explained more than 71% of the variation in estimates obtained by sampling all shoots on the branches. Sampling defoliation on a branch in whorls 1 and 2 (i.e., the leader and one branch in each of whorls 1 and 2) to evaluate tree-level defoliation explained more than 74% of the variation in defoliation in the two whorls combined. Cardinal direction influenced neither defoliation nor P. alaskensis density. Sampling the three most distally located one-year-old shoots of branches in whorls 2 and 4 accounted for more than 88% of the variation in P. alaskensis densities when all shoots on the branch were examined. Due to prolific dispersal by late-instar larvae from mid- and lower- to apical upper-crown branches, the leader, one whorl 1, and one whorl 2 branch were selected as the most appropriate sample unit for these instars. Relationships between the density of eggs and mid-instar larvae in whorls 2, 4, or both whorls combined, explained 21 to 49% of the variation in the density of late-instar larvae in whorls 1 and 2. Sampling branches in both whorls 2 and 4 may be necessary to account for seasonal variations in the distribution of eggs and/or mid-instar larvae within the crown of black spruce. Sampling methods provided by this study should facilitate the establishment of monitoring programs for P. alaskensis and associated defoliation in black spruce stands.