ECOLOGY AND POPULATION BIOLOGY Assessing the Threat Posed by Indigenous Exotics: A Case Study of Two North American Bark Beetle Species

The Douglas-Þr beetle, Dendroctonus pseudotsugae Hopkins, was detected in 2001 in northernMinnesota outside its natural range and the range of its native hosts, Douglas-Þr, Pseudotsuga menziesii (Mirbel) Franco, and western larch, Larix occidentalisNutt. Consecutive years of detection indicated that D. pseudotsugae may have been established in a new environment and provided a possible example of an indigenous exotic species in North America. Pheromone-baited logs of P. menziesii and tamarack, Larix laricina (Du Roi) K. Koch. were placed at four sites in northern Minnesota in an attempt to detect this indigenous exotic. Bark was removed from one half of logs and checked for D. pseudotsugae galleries. The remaining logs were left intact and reared to collect adult beetles the following spring. No D. pseudotsugae galleries were identiÞed and no adults of the indigenous exotic were located. Along with the eastern larch beetle, Dendroctonus simplex LeConte, native Minnesota populations of Polygraphus rufipennis (Kirby) and Dryocoetes autographus (Ratzeburg) colonized P. menziesii logs. The reciprocal study was conducted in Montana, primarily to determine whetherD. pseudotsugaewould attack pheromone-baited logs of L. laricina. Logs from the Minnesota andMontana experiments were used to estimate the success ofD. simplex andD. pseudotsugae in the non-native hosts P. menziesii and L. laricina, respectively. Both D. simplex and D. pseudotsugae successfully colonized and reproduced in non-native logs in the Þeld, albeit at low numbers. The potential threat of indigenous exotic species toNorthAmerican forests also is discussed.