title: Aggregation and a strong Allee effect in a cooperative outbreak insect.
abstract: 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.
authors.affiliation: Department of Biological Sciences, University of Alberta, CW 405, Biological Sciences Bldg, Edmonton, Alberta, T6G 2E9, Canada.
authors.forename: D W
authors.initials: DW
authors.lastname: Goodsman
authors.affiliation: Mathematical and Statistical Sciences, University of Alberta, 632 CAB, Edmonton, Alberta, T6G 2G1, Canada.
authors.forename: D
authors.initials: D
authors.lastname: Koch
authors.affiliation: Operations Division, Alberta Agriculture and Forestry, Peace River, Alberta, T8S 1T4, Canada.
authors.forename: C
authors.initials: C
authors.lastname: Whitehouse
authors.affiliation: Department of Biological Sciences, University of Alberta, CW 405, Biological Sciences Bldg, Edmonton, Alberta, T6G 2E9, Canada.
authors.forename: M L
authors.initials: ML
authors.lastname: Evenden
authors.affiliation: Canadian Forest Service, Northern Forestry Centre, 5320 122 Street Northwest, Edmonton, Alberta, T6H 3S5, Canada.
authors.forename: B J
authors.initials: BJ
authors.lastname: Cooke
authors.affiliation: Department of Biological Sciences, University of Alberta, CW 405, Biological Sciences Bldg, Edmonton, Alberta, T6G 2E9, Canada.|Mathematical and Statistical Sciences, University of Alberta, 632 CAB, Edmonton, Alberta, T6G 2G1, Canada.
authors.forename: M A
authors.initials: MA
authors.lastname: Lewis
mesh_terms: D000818:Animals; D001517:Coleoptera; D028223:Pinus; D011157:Population Dynamics; D014197:Trees; D048889:Weevils
keywords: Allee; Dendroctonus ponderosae Hopkins; dispersal; insect; integrodifference; management; model; mountain pine beetle; outbreak; parasitoid; population dynamics