Direction of interaction between mountain pine beetle (Dendroctonus ponderosae) and resource-sharing wood-boring beetles depends on plant parasite infection
Plant pathogens can have cascading consequences on insect herbivores, though whether they alter competition among resource-sharing insect herbivores is unknown. We experimentally tested whether the infection of a plant pathogen, the parasitic plant dwarf mistletoe (Arceuthobium americanum), on jack pine (Pinus banksiana) altered the competitive interactions among two groups of beetles sharing the same resources: wood-boring beetles (Coleoptera: Cerambycidae) and the invasive mountain pine beetle (Dendroctonus ponderosae) (Coleoptera: Curculionidae). We were particularly interested in identifying potential mechanisms governing the direction of interactions (from competition to facilitation) between the two beetle groups. At the lowest and highest disease severity, wood-boring beetles increased their consumption rate relative to feeding levels at moderate severity. The performance (brood production and feeding) of mountain pine beetle was negatively associated with wood-boring beetle feeding and disease severity when they were reared separately. However, when both wood-boring beetles and high severity of plant pathogen infection occurred together, mountain pine beetle escaped from competition and improved its performance (increased brood production and feeding). Species-specific responses to changes in tree defense compounds and quality of resources (available phloem) were likely mechanisms driving this change of interactions between the two beetle groups. This is the first study demonstrating that a parasitic plant can be an important force in mediating competition among resource-sharing subcortical insect herbivores.
KeywordsPlant defense Plant-mediated interactions Plant pathogen Plant-pathogen–insect interaction Species-specific traits
Funding for this research was provided by the Alberta Innovates–New Faculty Award, Canada Research Chairs program, and NSERC Discovery to N. E., as well as Alberta Innovates-Technology Futures, the Vanier Canada Graduate Scholarship, and the Izzak Walton Killam Memorial Scholarship to J. G. K. L. Barnhardt and D. Letourneau from Alberta Environment and Sustainable Resource Development helped with site selection. S. Taft, A. Sturm, I. Lusebrink and J. Therrien contributed to field experiments. We would like to thank J. F. Cahill, J. Karst, C. Tabacaru (University of Alberta) and two anonymous reviewers for valuable inputs that greatly improved the manuscript. The authors declare that they have no conflict of interest.
Author contribution statement
J. G. K. and N. E. conceived and designed the research experiments and wrote the paper; J. G. K. and A. N. conducted the chemical analysis; J. G. K. and J. A. C. analyzed the data; all authors provided editorial advice.
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