Survival of elongate hemlock scale (Hemiptera: Diaspididae) with prolonged cold exposure: overwintering mortality risk across North America.

Elongate hemlock scale, Fiorinia externa Ferris (Hemiptera: Diaspididae), is an invasive pest of eastern hemlock, Tsuga canadensis, and other Pinaceae such as Fraser fir, Abies fraseri. Cut Fraser firs (ie for Christmas trees and other holiday greenery) with F. externa have been intercepted in US states beyond where the insect is known to be established and, in some cases, where exposure to freezing temperatures might be prolonged. This study measures the effect of constant low temperatures (3, -10, or -20 °C) on survival of overwintering F. externa females from eastern hemlock in Michigan and from eastern hemlock and Fraser fir in North Carolina. Survival was determined with biochemical viability testing, which assesses the presence of metabolically active cells, and changes in survivorship through time were quantified with Kaplan-Meier methods and beta regression. Collection date and host occasionally affected survival rate but not in a consistent pattern. Survival rates generally decreased as temperature decreased. At 3 °C, F. externa maintained high survival (>75%). At -10 °C, survival was projected to fall to 50% within 22 to 92 d and 10% by 45 to 195 d. At -20 °C, survival typically declined to 50% by 1 to 27 d and 10% by 6 to 52 d. We used survival durations at -20 °C to map how often F. externa might experience 50 or 90% mortality from prolonged cold exposure. Temperatures in recent winters have not been below -20 °C for long enough to stop F. externa from spreading throughout the ranges of eastern hemlock or other hosts in Canada and the continental United States.