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The effects of fertilizer treatments on the resin canal defenses of spruce and incidence of attack by the white pine weevil, Pissodes strobi vanAkker, Lara

Abstract

The white pine weevil, Pissodes strobi (Peck), is a serious pest of regenerating spruce (Picea spp.) in British Columbia. On the coast, damage by this weevil results in such severe stem defects and growth losses in Sitka spruce (Picea sitchensis), that planting this species is not currently recommended in high weevil hazard areas. In the interior of the province, hundreds of millions of interior spruce seedlings (Picea glauca x englemanii) are currently in weevil susceptible age classes. Past attempts to control this weevil have been unsuccessful. Current research is focused on identifying trees with genetic resistance to the weevil and identifying the defense traits which give rise to this resistance. While weevil resistance has been shown to be heritable, environmental factors such as plant nutrient status may alter the expression of defense traits. My goal was to determine the effects of fertilization treatments on the resin canal defenses in spruce and incidence of weevil attack. Fertilizer effects on tree size variables were measured to provide baselines for the comparison of treatment groups and to facilitate discussion on the effects of variation in tree size due to fertilizer treatment, on incidence of weevil attack. The constitutive resin canal systems of Sitka and interior spruce maintained under several fertilizer regimes were compared by histological examination of leader cross sections. Traumatic response intensity was compared utilizing mechanical wounding to stimulate a traumatic response, followed by histological examination of cross sections through the wounded stem. Incidence of weevil attack in response to fertilizer treatments was studied in a caging experiment in which weevils were given a choice between trees from different fertilizer treatments, and in a variably fertilized plantation, naturally infested with weevils. The effects of fertilization on constitutive resin canal variables were influenced by spruce genotype, but not resistance status. In general, there was a dilution in cortical resin canal defenses in response to fertilization, which included a decrease in resin canal density and an increase in depth and distance between inner resin canals. These effects corresponded with, and may have been due to increasing bark thickness in response to fertilization. In severely nutrient stressed trees, fertilization may also improve the ability of trees to produce a traumatic response. Fertilization increased the incidence of weevil attack in both Sitka and interior spruce. In interior spruce, more than twice as many trees were attacked in the intense fertilizer treatments than in the unfertilized control groups. The observed increase in weevil attack in the more intense fertilizer treatments may be attributed to increased host vigor, resulting in an increase in resources available for weevil feeding and oviposition and a dilution of cortical resin canals. It is recommended that further studies be undertaken to investigate the effects of fertilization on other factors such as resin composition and flow.

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