HISTOLOGICAL RESPONSE OF RESISTANT AND SUSCEPTIBLE WHITE SPRUCE TO SIMULATED WHITE-PINE WEEVIL DAMAGE

The traumatic wound response of families of white spruce, Picea glauca (Moench) Voss, resistant or susceptible to the white pine weevil, Pis sodes strobi (Peck), were compared after simulated weevil damage. Lead ers from 331 trees were wounded just below the apical bud in the sprin g, coinciding with the natural time of weevil oviposition. A portable 1-mm diameter drill was used to drill 24 holes per leader. Leaders wer e removed in the fall and examined for evidence of traumatic resin can al formation. Drilled trees had a traumatic wound response 8 times gre ater than that of undrilled trees; however, undrilled trees also forme d some resin canals in response to unknown causes. In the drilled tree s, the traumatic wound response extended into the lower part of the le ader, where it could possibly affect older larvae. Trees from resistan t families responded with greater intensity than trees from susceptibl e families, by producing multiple rings of traumatic resin canals. Tre es from resistant families also responded more rapidly than trees from susceptible families based on number of cells to the first ring of tr aumatic resin canals. Trees from some resistant families exhibited no traumatic resin canal formation, showing considerable within-family va riation and suggesting that other resistance mechanisms might be impor tant. In the year after drilling, there was a reduction in tree diamet er growth and trees suffered a reduction in constitutive resin canals in the bark, which suggests some energetic cost of traumatic resin pro duction. There was no indication that the extent of constitutive defen ses, as measured by density of cortical resin canals before wounding, was related to the ability to produce traumatic resin canals. Screenin g trees based on their capacity to produce traumatic resin canals may be useful in selecting genotypes resistant to white pine weevil.