Influence of pre-commercial thinning of balsam fir on defoliation by the balsam fir sawfly

Pre-commercial thinning, whereby tree densities are reduced to diminish competition and maximize tree growth, is one of the most frequently used silvicultural practices in North America. We carried out field surveys in western Newfoundland and in eastern Nova Scotia, Canada, in pre-commercially thinned and unthinned stands of balsam fir (Abies balsamea (L.) Mill.) that had been defoliated by the balsam fir sawfly, Neodiprion abietis (Harr.), to determine if pre-commercial thinning increased the susceptibility of stands to insect defoliators. Except for stands sampled at the new and increasing stage of an outbreak, both egg densities of, and defoliation by, N. abietis were higher in thinned than unthinned stands. Higher levels of defoliation in thinned than unthinned stands were usually associated with higher levels of defoliation on intermediate-aged foliage. An estimate of tree vigor, used to predict future tree growth rate, was only weakly related to defoliation levels and it is, therefore, uncertain if higher defoliation in thinned stands would result in lower future growth rates than for trees in unthinned stands.