Density-dependent responses of mountain pine beetle to its pheromones and host volatiles in naïve lodgepole pine stands

Abstract

Mountain pine beetle has expanded its host range into naïve lodgepole pine forests in western Canada and killed a large number of pine trees. Synthetic attractants mainly bark beetle pheromones and host monoterpenes attached to traps are typically used to monitor beetle populations within its range. Despite well-established differences in beetle host selection behavior between endemic and outbreak population densities, the influence of population density on the response of mountain pine beetle to the synthetic attractants is poorly understood. Over three years, we tested the effects of different release rates of female mountain pine beetle aggregation pheromone (trans-verbenol) and two host monoterpenes (terpinolene and mycrene) and local beetle population density on trap catches of mountain pine beetle. We estimated local mountain pine beetle population densities using the number of dead trees observed in the previous year’s aerial detection survey. During the first three weeks of the flight period, the lures with low release rates of all components captured more mountain pine beetle with increasing beetle population density and the lures with a high release rate of trans-verbenol plus a low release rate of host monoterpenes were preferred by beetles in low population density sites. We also found a temporal shift in attraction patterns. As the flight period progressed, the preference for low trans-verbenol release rate lures was less pronounced at high mountain pine beetle densities. No sex-specific responses of mountain pine beetles, along with number of competitor and predatory beetles of mountain pine beetle, were found for a particular attractant. These results provide evidence that mountain pine beetle has a density dependent response to attractants and can assess available semiochemicals cues to ensure successful host colonization depending on their density and the flight period. Overall, this study can improve the efficacy of current semiochemical-based monitoring tools, which are essential to implementing informed mitigation strategies for mountain pine beetle especially while they are in a sub-outbreak stage in the expanded distribution.