Density-mediated responses of bark beetles to host allelochemicals: a link between individual behaviour and population dynamics

1. Bark beetles (Coleoptera: Scolytidae) accept or reject host conifers based partly on concentrations of phloem monoterpenes. They colonise trees in aggregations, in response to pheromones that attract flying beetles to trees undergoing colonisation. A series of entry and gallery construction assays was conducted to determine whether responses by individual beetles to monoterpenes are altered by pheromones and/or the presence of other beetles. 2. Entry into the amended media by Ips pini and the length of time until entry were not influenced by the presence of aggregation pheromones. 3. Entry into amended media was influenced by the presence of other beetles on the surface of, or constructing galleries in, the substrate. The effects of alpha-pinene and limonene on host entry behaviour were mediated by the density of beetles on the surface of the assay arena, and by the density of beetles constructing galleries within the medium. 4. The percentage of beetles entering medium amended with higher concentrations of monoterpenes increased with increased density of beetles on the surface of the assay arena, until a threshold density of three or four beetles per assay arena, after which entrance rate declined. 5. The presence of other beetles constructing galleries elicited more rapid entry by the test beetles. 6. Gallery lengths were generally higher in the presence of aggregation pheromones. 7. Gallery lengths increased with increased density of beetles within the assay arena. 8. These results suggest a link between the density of bark beetles and responses of individuals. This linkage may partially explain behavioural changes observed during population eruptions.