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. Study Implications 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.

The role of height‐driven constraints and compensations on tree vulnerability to drought

“…This size dependency is however host and biotic‐agent species‐specific and can even shift between normal and outbreak conditions (Raffa et al ., 2008; Kolb et al ., 2016). While bark‐feeding insects may favour larger trees due to their thicker phloem, most pest insects (including bark beetles) preferentially attack smaller trees as they are often suppressed and hence have a lower defensive capacity (Boone et al ., 2011; Gaylord et al ., 2013; Nahrung et al ., 2014; Takagi et al ., 2018; Martinson et al ., 2019; Davis et al ., 2022). Taller trees have higher LMA (Fig.…”

Using Remote Sensing and Climate Data to Map the Extent and Severity of Balsam Woolly Adelgid Infestation in Northern Utah, USA

“…The balsam woolly adelgid (Hemiptera: Adelges picea Ratzeburg; hereafter BWA) is an introduced insect that affects North American true firs (Abies spp. ), causing tree damage and mortality among all age classes [1]. Native to Europe, BWA entered North America in the early 1900s via separate introductions on the east [2] and west coasts [3].…”

“…Unlike these insects, which tend to produce relatively rapid and spatially clustered changes in forest canopy conditions and associated spectral responses, the complex spatiotemporal nature of BWA infestation has resulted in a relative paucity of remote-sensing-focused BWA studies. For example, tree health impacts from BWA infestation can unfold over the course of several years, and although mortality may be an eventual outcome, it is not guaranteed [1]. Furthermore, the spatially diffuse nature of infestation attributed to wind-driven insect dispersal increases the likelihood that a diverse range of infestation severities may be present even within a single satellite image pixel of moderate spatial resolution (e.g., 30 m), obscuring the stand-level spectral signal of infestation [16,32].…”