Outbreaks of metallic wood borers (Coleoptera: Buprestidae) cause widespread tree mortality which profoundly alters forest communities. For example, bronze birch borer (Agrilus anxius Gory) killed over 100 million birch (Betula spp.) trees in the Great Lake region following the 1988-89 drought. Wood-borer outbreaks are assumed to occur when trees are unable to resist colonization because defenses have been weakened by stress or because they are absent all together. Birch exhibit substantial interspecific variation in resistance to bronze birch borer. North American species such as paper birch (B. papyrifera Marsh.) are resistant, but exotic species like European white birch (B. pendula Roth) that lack a coevolutionary history with bronze birch borer are highly susceptible. The mechanism of this resistance is not understood. We hypothesize that native birch species employ a combination of physical and chemical defenses that enable them to resist infestation. To test this hypothesis, we compared the rate of wound periderm formation (callus tissue induced by feeding injury to encapsulate larvae) and patterns of constitutive phenolic compounds (defensive chemicals produced secondarily during plant metabolism) in phloem tissues of native paper birch to European white birch.