Relationships between yellowheaded spruce sawfly, Pikonema alaskensis, density and defoliation on juvenile black spruce

Manipulative field experiments and field surveys were carried out to evaluate the relationship between the density of yellowheaded spruce sawfly, Pikonema alaskensis (Roh.), and resultant defoliation on young open-grown black spruce, Picea mariana ([Mill.] B.S.P.), in central Newfoundland. In sleeve-cage experiments, the number of early and late-instar larvae per current-year shoot explained greater than 73 and 69%, respectively, of variation in mid-crown branch defoliation and 34–75% of variation in leader defoliation. In field surveys, densities of eggs, mid-, and late-instar larvae in whorls 2 and 4 explained greater than 34, 46, and 75% of variation, respectively, of defoliation in leaders and in whorls 1 and 2 of black spruce. Estimates of adult female and male abundance obtained from sticky traps explained 66 and 40% of variations, respectively, in defoliation among trees within stands and almost 90% of variations in defoliation among stands. Relationships were slightly improved by incorporating previous defoliation into analyses as a covariate. Our results indicate that density–defoliation relationships for all stages of P. alaskensis on black spruce are robust and suitable for incorporation into a management program for this pest.