The Potential Role of Ectomycorrhizal Fungi in Determining Douglas-fir Resistance to Defoliation by the Western Spruce Budworm (Lepidoptera: Tortricidae)
Journal of Economic Entomology
There is phenotypic variation among individual trees of interior Douglas-Þr (Pseudotsuga menziesii var. glauca [Beissn.] Franco) in their resistance to defoliation by the western spruce budworm (Choristoneura occidentalis Freeman). We evaluated the potential role of ectomycorrhizal fungi in determining this resistance using half-sib seedlings derived from parent trees that are resistant versus susceptible to budworm defoliation in the Þeld. The seedlings were inoculated with Laccaria bicolor
... Laccaria bicolor ectomycorrhizal fungi, fertilized, or untreated. Approximately 48 d after treatment, late-instar larvae from a nondiapausing laboratory colony of C. occidentalis were allowed to feed on pairs of resistant versus susceptible seedlings for 1 wk. Chemical analyses of current-year shoots for nitrogen (N), phosphorus (P), magnesium (Mg), and zinc (Zn) indicated that the fungus increased foliar concentrations of P and Mg in resistant seedlings, but it did not increase their growth rate. However, L. bicolor had no effect on foliar concentrations of P or Mg in susceptible seedlings, even though seedling growth rates increased slightly in response to the inoculation. L. bicolor had no effect on foliar levels of N or Zn in any of the seedlings. As expected, fertilization increased levels of N and P in the foliage of both resistant and susceptible seedlings, but it did not affect levels of Mg and Zn. Surprisingly, the fertilizer treatment had no effect on seedling growth rates. Despite these differences, late-instar budworms showed no feeding preference among untreated, mycorrhizal, or fertilized seedlings. The fact that seedlings from resistant versus susceptible Douglas-Þrs responded differently to the L. bicolor treatment lends preliminary support to the hypothesis that ecotmycorrhizae might play a role in Douglas-Þr resistance to damage from the western spruce budworm. Finally, differences in foliar concentrations of N and P among untreated seedlings from different maternal trees suggested that foliar nutritional chemistry is inßuenced by the treeÕs genotype.