Characterizing Balsam Woolly Adelgid Infestations and Associated Tree Mortality in Idaho

Abstract

Balsam woolly adelgid, Adelges piceae (BWA), is a nonnative insect that has invaded much of the true fir range in North America, reducing host tree growth and causing mortality. Here, we describe a field study to characterize BWA infestation and effects on host trees in Idaho from the onset of infestation within twenty-eight sites with repeated surveys in 2008, 2013, and 2018. Declining tree health was categorized into five BWA damage classes based on the percent of the tree crown–expressing symptoms caused by BWA. Between 2008 and 2018, BWA presence increased across most sites regardless of host species, with an average of 48% (0.1%–100%) host trees per acre being infested. Severe damage or mortality of true fir from BWA occurred across thirteen sampled forest habitat types, including those representing harsh, high elevations. Although a significantly greater proportion of subalpine fir died than grand fir by 2018, BWA caused grand fir mortality. All diameter size classes became infested with BWA, including seedlings, and BWA caused mortality regardless of tree diameter. This assessment provides baseline data for a deeper analysis of insect, host, stand, and environmental interactions that may elucidate factors driving severe stand mortality. Balsam woolly adelgid (BWA) is a cryptic, nonnative, and chronically lethal insect of true firs that is frequently overlooked until tree health substantially declines. Therefore, it is often not considered a serious damage agent during forest management planning until after severe damage has occurred across a stand. The BWA damage classification, modified from Spiegel et al. (2013), may be applied during stand exams. This simple and effective five-tiered classification system can be used to categorize BWA damage within individual trees or stands based on the collective symptoms caused by the insect. Feeding by BWA can ultimately result in stand-level mortality of both over- and understory fir and may require artificial regeneration to restore forest function. Fir regeneration was abundant at most BWA-damaged sites; however, seedling-sized firs were often heavily gouted from BWA to levels that will prevent them from growing into seed-bearing trees. Reported levels of mortality caused by BWA within 10 years of infestation and forest habitat type associations can be referenced for making management decisions for not-yet or recently infested stands.