Phenology of the invasive Adelges piceae in northern Utah

Adelges piceae (Ratzeburg), the balsam woolly adelgid, is a forest insect likely native to the Caucasus Mountains of Europe that has expanded into the Intermountain region of North America. This pest attacks true firs (Abies spp.), where it restricts water flow, causes crown dieback, and accelerates tree mortality. In northern Utah, the abundant high elevation subalpine fir, Abies lasiocarpa (Hook.) Nutt., is highly susceptible to A. piceae and a critical species for watershed health and wildlife habitat. Thorough understanding of the phenology of A. piceae in northern Utah is limited and imperative in supporting development of effective management strategies. In this study, bark samples of A. lasiocarpa infested with A. piceae were regularly collected from five sites in northern Utah ranging in elevation from 2,039 to 2,456 m from August to November, 2020 and January to November, 2021 and 2022. Adelges piceae life stages (egg, three nymphal instars, and adult) were enumerated and live adelgids distinguished from dead. Methods for improved accuracy and efficiency in sampling and sample processing were developed. Data were prepared as cumulative proportions for assessing patterns of voltinism including estimation of generation number and timing using a Weibull distribution. Results support a bivoltine life history with peaks around 515 and 1,750 DD₃ and minimal elevational variation in phenology in northern Utah. Our findings and improved sampling and processing methods will facilitate development of a degree day model that can inform timely pest management decisions to support the region’s watershed and forest health.