Temporal patterns of balsam fir sawfly defoliation and growth loss in young balsam fir

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A 2-year field study was carried out with the balsam fir sawfly, Neodiprion abietis (Harris), on balsam fir, Abies balsamea (L.) Mill., to determine: (1) the temporal patterns of defoliation and foliage weight loss associated with early- and late-larval instars; (2) the losses in branch and tree stem volume growth as a result of defoliation by larvae; and (3) to determine the extent to which individual branches are autonomous, and their utility for predicting stem growth losses due to defoliation. In addition, this study also permitted us to determine the relationships between defoliation or foliage weight and branch and tree stem volume growth. Defoliation, foliage weight loss and larval development occurred more quickly in the first year due to warmer temperatures, but final levels of defoliation and foliage weight loss were similar in both years. Cumulative defoliation levels increased from <5% at the beginning of the study to 35 and 63%, respectively, in fall 1999 and 2000. Defoliation was greatest on 1-year-old foliage in both years and most defoliation and foliage weight loss occurred after the initiation of third-instar. Based upon individual year–volume increment relationships for each branch and tree, mean volume increment loss in 1999 and 2000, respectively, was 20 and 42% for observation branches and 12 and 35% for tree stems. Similar growth loss estimates of 12 and 46%, respectively, for 1999 and 2000, were obtained when comparing the mean specific volume increment for control and observation branches. Foliage weight after sawfly attack was related to branch and stem volume growth, but defoliation was only related to branch volume loss. Although branch and tree stem growth were strongly related, growth losses in a branch were not related to those in a tree stem. Branch removal and measurement is a practical alternative to cutting trees to estimate stem volume growth in balsam fir.