Detecting Balsam Woolly Adelgid Infestations Using Tree Rings

Abstract

Balsam woolly adelgid (BWA), Adelges piceae(Ratzeburg), is a sap-sucking, exotic invasive insect that arrived in North America from central Europe around 1900. Since then, its range has expanded from New Brunswick, Canada to the southern Appalachian Mountains. It is a threat to all North American true-fir species, but populations can be controlled by cold continental winters. Adelgid feeding leaves noticeable traces on the wood tissue (“rotholz”) and causes a temporary positive radial growth response among affected trees. The purpose of this research was to determine dates of initial outbreaks, balsam fir radial growth change during outbreaks, and the relationship between cold winter temperatures and BWA outbreaks in the highlands of West Virginia. I used these two indicators on the wood tissue collected from tree cores from 14 balsam fir stands (Abies balsamea) and compared them to that of red spruce (Picea rubens) trees growing nearby. I also compared infestation dates with local climate records, to ascertain the relationships between infestations and winter air temperatures. I observed rotholz in 28% of the trees in all stands with the earliest evidence of occurrence at Upper Tract in 1952, four decades before BWA was first observed by the US-Forest Service at a Tucker County seed orchard. Using a host vs non-host model, I observed increases in growth caused by infestations particularly in 1988-1994 and 2000-2005. These periods are also periods of high rotholz occurrence. Periods of reduced infestation, as evidenced by normal or reduced growth and less rotholz, tend to occur following exceptionally cold winters. My results suggest that future monitoring of BWA should include tree core sampling for rotholz and growth increases in addition to exterior evaluation of trees. My results also suggest that as winters continue to warm, infestations may become more frequent and more severe.