Intra-Stand Movement of Balsam Woolly Adelgid (<i>Adelges piceae</i>) in the Intermountain West

Balsam woolly adelgid (BWA, Adelges piceae (Ratz.)) is an invasive insect of true firs (Abies spp.). BWA has spread from initial introductions on the west coast of North America in the 1920s through the Intermountain West and has recently invaded Utah. White fir (A. concolor) and subalpine fir (A. lasiocarpa) are the two main species of concern in this region; however, subalpine fir has been found to be more susceptible to attack and subsequent damage. While long-range dispersal mechanisms and patterns are of interest for spread across regions and for predicting new infestations, very little is known of BWA movement within a stand once it is present. We measured GPS locations of all trees within eight quarter-hectare BWA-infested subalpine fir stands in northern throughout Utah and southern Idaho and quantified BWA intensity on each tree at multiple points in time using a severity rating system. We measured tree- and stand-level attributes and quantified bole infestation levels twice per growing season for two seasons to detect change in infestation. We identified traits of individual subalpine fir individuals and stands that are associated with infestation status, change, and risk of new infestations. These models suggest that at a stand scale, individual tree attributes as well as larger scale neighborhood attributes affect the dispersal of BWA within subalpine fir stands. Our study suggests that current management recommendations should be followed and long-term monitoring should remain a priority.