Playing with fire: Dendroctonus ponderosae (mountain pine beetles) in post-burn lodgepole pine forests

• Author / Creator

  Tabacaru, Crisia A.

• Dendroctonus ponderosae, an aggressive tree-killing bark beetle, is one of the most significant insects in the coniferous forests of western North America. Although D. ponderosae is restricted to weakened host trees at low-density populations, fire can suddenly increase the number of suitable, less-defended hosts, and potentially allow increased beetle colonisation and population growth. However, it is unclear whether D. ponderosae is attracted to fire-injured hosts or whether post-burn forest stands can promote outbreaks. In this thesis, I address this and several related questions by examining D. ponderosae interactions with both fire and the broader subcortical insect community at three sites in the Rocky Mountains of Alberta, Canada. I showed that proportionally more fire-injured trees were attacked than non-burned trees every year for 4 years after fire, and that beetle density per tree was always greater on fire-injured trees. Next, I showed that prescribed fire likely does not promote outbreaks of D. ponderosae: at one of three sites, colonisation declined after an initial response to a resource pulse, and at the remaining two sites, colonisation remained low over time. Additionally, neither the proportion of trees attacked nor the number of attacks per tree increased in adjacent non-burned stands, which would have indicated local population increase associated with the impact of fire. However, I observed stabilisation of D. ponderosae colonisation 4 years after fire at one site, and ongoing low rates of attack at the remaining sites, potentially indicating that fires serve as refugia for low-density populations of D. ponderosae. Various factors appear to negatively impact D. ponderosae, interfering with its ability to take advantage of post-fire stands. First, although declining resource (phloem) quality likely does not explain why D. ponderosae cannot take advantage of a post-fire stand, I show that declining resource quantity, i.e., preferred trees dying soon after fire, probably affects the beetles negatively in their host-searching phase 1 year after fire. Second, fire-injured trees attract a diverse subcortical insect community, including competitors and predators of D. ponderosae. I show that this community, especially other bark beetles that compete directly with D. ponderosae, is associated with areas containing D. ponderosae. Thus, post-fire forest stands likely intensify interspecific interactions for D. ponderosae, especially competition. To test these interactions, I used passive traps to examine the subcortical communities present in burned and adjacent non-burned stands because using lures would have artificially aggregated insects and mixed samples. I compared trap efficiencies and found that landing traps, placed directly on host trees, catch more individuals per unit area than flight intercept traps. However, intercept traps catch a greater number of species, especially Hymenoptera. Overall, I show that although low-density populations of D. ponderosae can use post-fire stands as refugia from extirpation until new weakened host trees arise on the landscape, various factors interact to suppress potential population growth and outbreak.

• Subjects / Keywords

  • Dendroctonus ponderoae
  • Prescribed fire
  • Subcortical insect community

• Graduation date

  Spring 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3KD1QS2M

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.