Dendrochronological Reconstruction of the Historical Invasion of Balsam Woolly Adelgid, Adelges piceae, Feeding on Canaan Fir, Abies balsamea subsp. phanerolepis in the Central Appalachian Mountains
During biological invasions, the initial arrival and establishment of invading populations often go unnoticed for many years yet information on early invasion dynamics is key to understanding and managing invasions. We used the presence of ring discoloration, “rotholz”, in tree cores to date historical Adelges piceae outbreaks and reconstruct its invasion history in 14 Abies balsamea subsp. phanerolepis stands in West Virginia. In 2018, we collected and cross-dated tree ring samples from 676 cores. We measured ring width increments and recorded the presence of rotholz in each annually dated ring. Rotholz was present in tree cores sampled from each of the 14 study sites, with the first rotholz occurrence in 1952 and widespread evidence of rotholz as early as the 1980s. Time series of rotholz frequencies indicate two synchronized waves of elevated rotholz frequencies that were observed at most sites from 1998 to 2003 and again from 2006 to 2013, coincident with observed outbreaks. Unlike patterns of growth suppression found among trees affected by defoliating insects, we observed a significant and positive relationship between rotholz frequency and standardized growth increment. This relationship is consistent with earlier observations of abnormal growth among trees infested by A. piceae. Our samples of rotholz frequencies suggests that rotholz may be a better indicator of past outbreaks of A. piceae than growth. The presence of A. piceae and its role in hastening decline of these rare relic tree populations would lend support to increased attention for the search for A. piceae biological control agents to limit damage by this insect.