North American forests have been devastated by invasive organisms such as Dutch elm disease, chestnut blight, and the emerald ash borer. When an invasive species threatens to extirpate its host plant, one important management response should involve searching invasive-devastated areas for surviving plants that may possess some degree of resistance/tolerance to the invader. This approach has previously yielded evidence for naturally-occurring resistance to pathogens such as butternut canker and Dutch elm disease. The search for such survivors needs to be combined with efforts such as quarantines, pest eradication, and biological control efforts in order to optimize conservation efforts. The more devastating a pest is, the more likely it is that the surviving hosts truly possess some level of resistance/tolerance; ironically, however, traditional approaches to invasive organisms such as pre-emptive logging may inadvertently remove these rare host individuals and ultimately reduce the probability of forest recovery. When devastating invasions occur, we propose that a standardized approach for identifying and evaluating surviving host plants should be an integral part of management or conservation plans.

Results/Conclusions

We illustrate this approach using an ongoing initiative aimed at locating naturally-occurring eastern hemlock trees (Tsuga canadensis) which may be resistant to the hemlock woolly adelgid (Adelges tsugae, ‘HWA’). HWA is an invasive insect pest of eastern hemlocks that has spread throughout the eastern United States and causes stand-level tree mortality. Our efforts to locate potentially resistant/tolerant trees began with the development of a protocol for identifying such individuals in forested landscapes. After developing guidelines for identifying potentially resistant/tolerant trees, we developed a brochure detailing these criteria and distributed it widely throughout the current range of HWA infestations. The brochures generated considerable interest and local newspaper coverage, and resulted in the discovery of 11 sites with one or more potentially resistant/tolerant mature hemlock trees. We have taken and propagated cuttings from each candidate tree (and from control trees) in a greenhouse setting. Once the cuttings have developed root systems, we plan on exposing them to HWA in order to assess their degree of resistance/tolerance. Our approach to the search for surviving hemlock trees can easily be coupled with ongoing infestation surveys or stand-level monitoring efforts, making it an effective and low-cost complement to current techniques aimed at mitigating the impact of invasive species.