Balsam woolly adelgid, Adelges piceae (Ratz.) (BWA), invasive in North America, was first detected on subalpine fir [Abies lasiocarpa (Hook.) Nutt.] in several northern Utah counties in 2017. BWA phenology is known to vary by elevation and climate; a degree-day (DD) model focused on population expansion into the Intermountain West is needed. Bark samples were collected weekly from infested subalpine fir in early summer through late fall at 5 northern Utah sites from August 2020 to December 2022. At a single site, additional samples were collected biweekly to monthly during winter and spring.The presence of live individuals of all life stages in winter through early summer samples confirmed that life stages other than crawlers can successfully overwinter in northern Utah. Two generations were observed at all sites. Degree-day models were developed by fitting proportional adult counts and local air temperatures to Weibull distributions. Model fit was optimized with a 0 °C lower threshold, 30 °C upper threshold, and 1 Jan biofix. Completion of the first generation required 1,104 DD and 2 generations required 2,412 DD. Using the models and historical (1980 to 2020) temperatures, study sites were predicted to have thermal suitability for 2 generations at least 2 to 3 decades prior to detection in northern Utah, depending on site. Although upper estimates of future (2025 to 2099) predictions forecast a doubling of generations by 2060, knowledge of potential dormancies that may be disrupted in a changing climate is needed.The degree-day model will be a useful tool for predicting thermal suitability for future BWA expansion.

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