Population dynamics of Ips pini and Ips grandicollis in red pine plantations in Wisconsin: Within- and between-year associations with predators, competitors, and habitat quality

We sampled beetle (Coleoptera: Scolytidae) populations in 17 declining and healthy red pine plantations in Wisconsin over 3 yr. We tested for potential relationships among numbers of bark beetles, conspecifics and competitors, and predators within and among flight seasons to help identify factors affecting population densities. The two most common bark beetle species obtained were Ips pini (Say) and Ips grandicollis (Eichhoff). The predominant predators obtained were Thanasimus dubius (F.) (Cleridae), Platysoma cylindrica (Paykull) (Histeridae), and Platysoma parallelum Say. Declining stands contained significantly more Ips than did healthy stands during the early portion of the season. Healthy stands had more predators than declining stands. There were strong delayed inverse relationships between I. pini and predators at the site level, both within and between flight seasons. The number of I. pini caught during the late portion of the season was lower when each of the above predators was more abundant earlier in the season, during both 1998 and 1999, Likewise, numbers of I. pini and I. grandicollis caught during the early portion of the year were inversely related to numbers of predators caught during the previous year. Although Ips trap counts showed significant correlations with each predator species, simple predator-prey models did not necessarily improve fits based on habitat quality (i.e., Ips numbers regressed on prior Ips numbers). We did not observe evidence for interspecific competition among Ips spp. This pattern is consistent with the view that host plant quality and predation jointly affect I. pini and I. grandicollis population dynamics. These results, emphasize the importance of interactions among host tree physiology, predation, and dispersal in the population dynamics of phloeophagous herbivores, and have implications to forest management.