Canadian Forest Service Publications
Characterisation of attacks made by the mountain pine beetle (Coleoptera: Curculionidae) during its endemic population phase. 2013. Bleiker, K.P.; O'Brien, M.R.; Smith, G.D.; Carroll, A.L. The Canadian Entomologist 00: 1–14
Issued by: Pacific Forestry Centre
Catalog ID: 35449
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Mountain pine beetle (MPB) Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae) attacks and overwhelms the defences of vigorous trees during outbreaks by attacking en masse. Low or endemic populations are regulated by host resistance and restricted to colonising weakened trees, where there is a potential trade off between tree defences and habitat quality. Mountain pine beetle populations are typically in the endemic population phase, but MPB attack behaviour and brood productivity in this phase are poorly understood. We located attacks made by beetles from endemic populations in north-central Alberta, Canada and examined galleries constructed on these trees. The distribution of gallery starts on trees was clustered relative to height on the tree, but not related to aspect on the tree bole. We found no Allee effect associated with mate location as over 99% of galleries were constructed by mated females. Productivity was generally low and brood development rarely reached the pupal stage, with one exception that suggests that endemic populations are capable of rapid increase in certain hosts. Egg galleries constructed by unmated females differed in morphology from galleries created by mated females. To understand the dynamics of this eruptive species, we need to identify the conditions under which endemic populations can persist and periodically increase to densities that result in coordinated mass attacks on healthy trees and lead to outbreaks.
Plain Language Summary
During outbreaks, mountain pine beetle is able to kill healthy trees, overwhelming their toxic chemical defenses by attacking en masse (cooperative group attack). In between outbreaks, when population levels are low (i.e., endemic level), beetles must survive in stressed trees, which have reduced defenses, but such trees are also poorer quality hosts for the beetle. Most of our current knowledge on mountain pine beetle comes from outbreak populations. This study located attacks made by beetles from endemic populations and describes the characteristics of these attacks. In contrast to attacks made by beetles from outbreak populations, the distribution of gallery (attack) starts was not related to aspect on the tree bole. We demonstrated that the galleries made by mated and unmated females could be differentiated by examining their shape. Despite some trees only having one or two attacks, over 99% of the galleries were constructed by mated females. This means male beetles were able to locate female beetles despite population levels being very low. Although most females were mated, reproductive success was extremely low. However, there was one exception that was found, which demonstrates that endemic populations are capable of rapid increase in certain host trees. To fully understand the dynamics of this eruptive species, we need to identify the conditions under which endemic populations can increase to a level where they are able to coordinate mass attacks and kill healthy trees.