A multiple equilibria model for Dendronctonus frontalis which includes predation and competition

Southern pine beetle (Dendroctonus frontalis) populations exhibit extreme fluctuations and are arguably the greatest biotic disturbance in U.S. forests. When D. frontalis populations are rare, beetles are so difficult to find that it may be arguable that they are locally extinct.  It is puzzling to consider how populations move from being very rare, to extremely abundant.  We present data showing a bi-modal population abundance distribution, suggesting both a regulated, endemic population state for D. frontalis, as well as the outbreak state. We test a multiple equilibria model for this system, with a regulating mechanism based on competition with Ips beetles and predation.  In pairs of lightning-struck trees (host material for rare D. frontalis) we test the hypothesis that competition and predation on trees baited with frontalin (aggregation pheromone of D. frontalis) would be higher than on trees with no pheromone. On frontalin-baited trees, competitors were somewhat more abundant, and predators were significantly more abundant.  The small increase in the number of competitors translated into a significant increase in use of host resources, suggesting that both predation and competition are stronger in these trees.  Additionally, we did field assays with different dosage levels of frontalin, mimicking different population levels of D. frontalis, and found that Ips beetles have no behavioral response to pure frontalin, but that the specialist predator, Thanasimus dubious does show a positive numeric response. These tests support our multiple equilibrium model, and provide evidence for a mechanism for D. frontalis populations to move between two alternative stable states.