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Seasonal variation in constitutive and induced defenses of spruce (Picea spp.) hosts of the white pine weevil Pissodes strobi peck Brescia, David A.

Abstract

The white pine weevil, Pissodes strobi, is a major pest of regenerating spruce in British Columbia, and the damage caused by weevil attack results in a severe reduction in plantation productivity and wood quality. As a result, the planting of Sitka spruce, except in a small portion of its range, is no longer recommended in B.C. Control attempts have met with little success; however, it is believed that host resistance will be an essential part of an effective Integrated Pest Management plan for the weevil. My objective was to examine the induced resin response and the constitutive resin canal system of Sitka spruce to identify potential resistance mechanisms and to determine how those mechanisms were affected by seasonal growth and development of the tree. Histological examination of cross-sections of laterals taken at different times during the season were used to compare the extent of components of the constitutive resin canal system between resistant and susceptible families of Sitka spruce. Similar methods were used to compare the capacity of resistant and susceptible trees to produce a traumatic resin response. Weekly measurements of leader apical bud phenology and radial growth were used to determine if differences in leader growth and development existed between resistant and susceptible trees which could affect resin canal density in the leader. Artificial wounding of tree leaders at different times of the season and at different densities of wounding was used to determine if there was a seasonal effect on the production of traumatic resin and if the response level was related to the wounding level. Compared to susceptible trees, resistant trees tended to have larger inner resin canals and thinner bark, and thus, a greater percentage of bark occupied by inner resin canals. Furthermore, both resistant and susceptible trees showed an increase in resin canal size and a decrease in resin canal density through the season. The level of the traumatic response produced by artificially wounded trees was significantly higher in resistant trees than in susceptible trees and was also found to be highest in early flushing trees. No difference in rate of leader radial growth was observed between resistant and susceptible trees, but buds on resistant trees flushed earlier than those on susceptible trees. The traumatic response was found to be lowest in trees wounded early in the season, and was also found to increase in a density dependent manner with the wounding level. Although several traits were present to a greater degree in resistant trees than susceptible trees, no single trait was emphasized in every resistant individual, suggesting that effective resistance is based on a combination of traits. Furthermore, the variability of these traits through the season reinforces the importance of host-insect phenology in the success or failure of an attack.

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