Characterisation of attacks made by the mountain pine beetle (Coleoptera: Curculionidae) during its endemic population phase

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.