Effect of varying monoterpene concentrations on the response of Ips pini (Coleoptera: Scolytidae) to its aggregation pheromone:: implications for pest management and ecology of bark beetles

1 Host plant terpenes can influence attraction of conifer bark beetles to their aggregation pheromones: both synergistic and inhibitory compounds have been reported. However, we know little about how varying concentrations of individual monoterpenes affect responses. 2 We tested a gradient of ratios of alpha-pinene, the predominant monoterpene in host pines in the Great Lakes region of North America, to Ips pini's pheromone, racemic ipsdienol plus lanierone. 3 Ips pini demonstrated a parabolic response, in which low concentrations of alpha-pinene had no effect on attraction to its pheromone, intermediate concentrations were synergistic and high concentrations were inhibitory. These results suggest optimal release rates for population monitoring and suppression programmes. 4 Inhibition of bark beetle attraction to pheromones may be an important component of conifer defences. At terpene to pheromone ratios emulating emissions from trees actively responding to a first attack, arrival of flying beetles was low. This may constitute an additional defensive role of terpenes, which are also toxic to bark beetles at high concentrations. 5 Reduced attraction to a low ratio of alpha-pinene to pheromone, as occurs when colonization densities become high and the tree's resin is largely depleted, might reflect a mechanism for preventing excessive crowding. 6 Thanasimus dubius, the predominant predator of I. pini, was also attracted to ipsdienol plus lanierone, but its response differed from that of its prey. Attraction increased across all concentrations of alpha-pinene. This indicates that separate lures are needed to sample both predators and bark beetles effectively. It also provides an opportunity for maximizing pest removal while reducing adverse effects on beneficial species. This disparity further illustrates the complexity confronting natural enemies that track chemical signals to locate herbivores.