Host Resistance to the Fir Engraver, Scolytus Ventralis (Coleoptera: Scolytidae): Frequencies of Attacks Contacting Cortical Resin Blisters and Canals of Abies Concolor

Observed and expected frequencies with which fir engraver ( Scolytus ventralis ) attacks contacted resin blisters or canals in the cortex of white fir ( Abies concolor ) stems were compared. Cortical regions of the stems were sampled by single bolts sawn from each of 24 firs which had been attacked after felling. Expected frequencies were based on mean proportions of bole surface occupied by the resin structures at attack sites on the bolt. Frequencies were compared on a per-bolt basis and results were combined for χ 2 tests. Blister contact (6.5% of attacks sampled) was less than expected (14.0%). Canal contact (17.4%) exceeded the expected (8.2%). Contact frequencies varied directly with percentages of bole surface occupied by the resin structures, and the latter varied significantly among sample bolts. Among-bolt variations in the resin structures were not related to diameters, radial growth rates, or ages of the sections of stem included in the bolts, except that bolts with greater diameters and radial growth rates tended to have lower percentages of bole surface occupied by canals. Almost all beetle attacks that contacted the resin structures failed to produce larval galleries. Results indicate that unless firs are more heavily blistered or canalled than those sampled, the cortical resin system alone is apparently inadequate to protect the trees against the beetles.