Plant module size influences the intra-tree distribution and abundance of a shoot-boring sawfly in young balsam fir

Field surveys were carried out to assess the effects of intra-tree variation in developing shoot length within and among crown levels on the density and abundance of the balsam shoot-boring sawfly, Pleroneura brunneicornis Rohwer (Hymenoptera: Xyelidae), in young balsam fir, Abies balsamea (L.) Mill. (Pinaceae). Overall, cardinal direction had no influence on shoot-borer density or abundance; however, the highest percentage and abundance of bored shoots occurred on intermediate-sized shoots within the crown (i.e., in the mid-crown and on the distal-lateral and medial-lateral shoots). Comparatively, few shoot borers occurred in the upper or lower crown levels, or on the relatively large terminal shoots within branches. This distribution appears indicative ofthe higher suitability of intermediate-sized shoots within hosts for either egg lay or larval performance. Results of this study aremost consistent with predictions ofthe‘optimal module size’ hypothesis,whichposits that herbivore responses to plantmodule size should reflect the balance oftradeoffsbetween utilizing relatively large, nutritious shoots vs.small,moreeasilyexploitedshoots.