Influence of tree growth rate, shoot size and foliar chemistry on the abundance and performance of a galling adelgid

1. The abundance and performance of the Eastern Spruce Gall Adelgid, Adelge s abietis, was evaluated on young, open-grown trees of White Spruce, Picea glauca, whose growth rates had been increased through fertilization or decr eased through root-pruning. 2. In general, gall densities were highest on control trees and on mid-crow n branches. Reduced galling success on fertilized trees was largely due to higher overwintering mortality of first-generation nymphs. Foliar magnesium , total tannin and total phenol contents were positively, and nitrogen and total monoterpene contents negatively, related to gall density and/or galli ng success. 3. Although short shoots were more abundant, shoot length was parabolically related to gall density. There was a non-significant parabolic trend betwe en shoot size and galling success and volume. The number and average size o f A. abietis females emerging from galls were positively related to gall vo lume, and realized fecundity was positively correlated to female size. Cons equently, fitness was approximately twice as high for nymphs colonizing int ermediate-sized than small or large shoots. 4. These results do not support the plant vigour or plant stress hypotheses . The results do, however, agree with predictions of the modified plant str ess hypothesis for sucking insects. It is speculated that Adelges abietis l acks the necessary resources for successful gall formation on small shoots and is unable to produce a stimulus large enough to induce gall formation o n large shoots.