Variation in the induced resin response of white spruce, Picea glauca, to attack by Pissodes strobi

The intensity of the traumatic resin response in white spruce, Picea glauca (Moench) Voss, to attack by the white pine weevil, Pissodes strobi Peck, was studied in trees with different severities of attack. The response level was highest in trees where the attacks failed, i.e., where eggs were laid but the brood was killed and no adults emerged. Successfully attacked trees had, on average, only 62% of the response intensity of trees with failed attacks. Response intensity in trees that had been subjected only to feeding was much lower, at 42% of the failed attack response. Healthy unattacked trees showed no or little traumatic resin response. Response intensity varied in a nonlinear fashion with the number of eggs laid, increasing rapidly from zero in healthy trees, being highest in trees having between 10 and 60 egg punctures, and progressively lower again in trees with higher numbers of egg punctures. For a given number of egg punctures, resistant white spruce trees had a consistently higher traumatic resin response than susceptible trees. The number of eggs laid on a leader was inversely related to the intensity of the traumatic resin response and to the timing of the attack. Fewer eggs were laid on leaders with high traumatic resin response or attack:ed late in the season than on leaders with lower resin response or attacked early in the season.