Canadian Forest Service Publications
Aggregation and a strong Allee effect in a cooperative outbreak insect. 2016. Goodsman, D.W.; Koch, D.; Whitehouse, C. Evenden, M.L.; Cooke, B.J.; Lewis, M.A. Ecological Applications 26(8):2621-2634.
Issued by: Northern Forestry Centre
Catalog ID: 37525
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Most species that are negatively impacted when their densities are low aggregate to minimize this effect. Aggregation has the potential to change how Allee effects are expressed at the population level. We studied the interplay between aggregation and Allee effects in the mountain pine beetle (Dendroctonus ponderosae Hopkins), an irruptive bark beetle that aggregates to overcome tree defenses. By cooperating to surpass a critical number of attacks per tree, the mountain pine beetle is able to breach host defenses, oviposit, and reproduce. Mountain pine beetles and Hymenopteran parasitoids share some biological features, the most notable of which is obligatory host death as a consequence of parasitoid attack and development. We developed spatiotemporal models of mountain pine beetle dynamics that were based on the Nicholson–Bailey framework but which featured beetle aggregation and a tree-level attack threshold. By fitting our models to data from a local mountain pine beetle outbreak, we demonstrate that due to aggregation, attack thresholds at the tree level can be overcome by a surprisingly low ratio of beetles per susceptible tree at the stand level. This results confirms the importance of considering aggregation in models of organisms that are subject to strong Allee effects.
Plain Language Summary
An outbreak of the mountain pine beetle has devastated forests in western North America in recent years. The beetles cluster and cooperate with each other to overcome tree defenses. Understanding the factors that influence the beetles’ ability to cluster will help forest managers to create strategies to combat the beetle outbreak. We used a range of field and laboratory data to produce models to describe the dynamics between the beetles and trees as the beetles attacked cooperatively to overcome the trees’ defenses. We found that, on average, four beetles per tree were needed to establish a beetle population in groups (stands) of trees, but 545 beetles per tree were needed to establish a population in individual trees. This difference is explained by the critical role of beetle clustering at the stand level in overcoming host defenses in individual trees. The results of this study imply that any factor that influences the ability of beetles to cluster– such as host tree chemistry or the application of olfactory repellents – could act as a lever for effective population management.