Xylem Resin in the Resistance of the Pinaceae to Bark Beetles

Xylem resin-a supersaturation of resin acids in terpenesappears to play a paradoxical role in the relation of the Pinaceae to tree-killing bark beetles. It has been suggested as the agent responsible for the susceptibility of this coniferous family to beetle attacks. And, at the same time, it has been linked with the ability of the Pinaceae to resist bark beetles. The hosts of treekilling bark beetles are nearly all Pinaceae-Pinus (pine), Abies (fir), Pseudotsuga (Douglas-fir), Picea (spruce), Tsuga (hemlock), and Larix (larch). Significantly, among conifers, xylem resin is most common and abundant in these Pinaceae. Bark beetles are found on other families of conifers, but they are usually considered of minor importance. Much experimental work has been done since the early reports of an apparent association of resin with resistance. Nearly all studies have dealt with attacking adult beetles; virtually no research has been directed at effects of resin on immature forms. Most work has been with Pinus and Dep'roctonus, with lesser attention to spruce, fir, Douglas-fir, and the other genera of bark beetles. This report summarizes the early findings, updates them with results of more recent reports and, in some cases, reinterprets these previous reports. This review deals only with bark beetles that attack living trees and with xylem resin, although in a few instances, it concerns resin-related chemicals as well. This review follows the approach of Painter (1951), who proposed that plant resistance to insects depends on one or more