Mountain pine beetle infestation risk: interactions of population phase, host vigour and spatial aggregation

Pine trees attacked by mountain pine beetles (Dendroctonus ponderosae ) exhibited positive spatial association up to 39 m. MPBs typically attacked trees in the immediate neighbourhood of a focal tree and aggregation declined with increasing distance. The patchy distribution of attacks may result from conspecific attraction of MPBs by means of aggregation pheromones. The spatial extent of the attraction of beetles to existing infestation centres increased with beetle population size. When infestation was severe, host depletion inhibited aggregation over large spatial scales. The proportion of infested neighbours and tree diameter were the most important factors determining infestation risk. Beetle population size also determines the vigour class of preferred hosts. When the population was small, beetles were restricted to trees with smaller diameters. Crown length reduced the risk. Beetles appear to balance losses due to a greater defence capacity in more vigorous trees with the advantages of greater phloem thickness and quality.