An evaluation of climate influences on balsam woolly adelgid infestations in Idaho

Balsam woolly adelgid (Adelges piceae) (BWA) was first reported in western North America in 1928 and continues to expand its range eastward into the Rocky Mountains, causing substantial damage and mortality in firs (Abies spp.) across the region. Climate is thought to influence BWA in multiple ways, yet few studies have addressed this region. Here we compared mean climate variables at 28 field sites in Idaho, USA, where stand and BWA conditions were measured in 2018. We evaluated BWA conditions using two field-based metrics, BWA presence and host damage. Using information from published studies from Canada, we found that winters were almost always warm enough to support BWA at the Idaho sites, and summers were always warm enough to support completion of at least one or two generations of BWA. Application of a previously published climate risk index indicated no relationship with observed BWA damage. We used statistical models to assess the influences of mean climate variables and stand characteristics on BWA presence and damage observed at our sites. Warmer summers or wetter conditions were associated with greater BWA presence and damage. At sites with both colder and drier conditions, BWA was recorded as present but damage levels were substantially reduced. Given that our Idaho sites are representative of climate conditions across most of the ranges of subalpine fir and grand fir in the western US, we suggest (with lower confidence) that mean climate variables, as evaluated in this study, may not limit the expansion of BWA across the ranges of these host species, but may limit the damage. Future studies that have more detailed information (for instance, annual resolution of insect observations or more detailed snowpack data) and more locations in additional regions and climates will increase confidence in our results.