Feedback between individual host selection behavior and population dynamics in an eruptive herbivore

We examined the role of population density in host selection behavior of an eruptive insect herbivore, the spruce beetle Dendroctonus rufipennis. We conducted field and laboratory experiments on spruce beetles from 29 endemic and eruptive populations in Alaska and Utah, USA, and Yukon, Canada. Beetles from both population phases colonized trees that had been felled to remove host defenses. However, only beetles from eruptive populations colonized defended, healthy trees. A series of laboratory assays using host-based media amended with varying concentrations of phytochemicals identified several factors affecting population-dependent responses to hosts. First, beetles were repelled by high concentrations of the predominant spruce monoterpene, alpha-pinene, but intermediate concentrations elicited entry and gallery construction. Second, heritability assays suggested high genetic variance of host selection behavior within populations, and between-population differences persisted following rearing in a common environment. Third, beetles from eruptive populations were less likely to enter medium amended with phytochemicals and constructed shorter galleries, which disagreed with our prediction and seemingly contradicted the field observations. However, fourth, beetle avoidance of high concentrations of alpha-pinene decreased with the addition of other beetles, and this effect was more pronounced among beetles from eruptive populations than among those from endemic populations. This interaction broadened the host range of eruptive beetles. We propose that such density-dependent behaviors can maintain heterogeneity among population phases and contribute to positive feedback in herbivore population dynamics. A conceptual model suggests how heritable and environmentally induced variation in host selection behavior can affect bimodal equilibria and numerical thresholds in eruptive species.