Evolution of increased cold tolerance during range expansion of the elongate hemlock scale Fiorinia externa Ferris (Hemiptera: Diaspididae)

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1. Alien species often face novel challenges to their spread and population growth. One critical hurdle often involves an organism's ability to tolerate environmental extremes characteristic of their invaded range. Although abiotic factors often determine range limits, there is less evidence for local adaptation in invasive organisms whose initial arrival and rapid population growth is separated by a lengthy lag period. 2. The invasive elongate hemlock scale Fiorinia externa feeds on Eastern hemlock, Tsuga canadensis, on the east coast of North America. Following its 1908 arrival, it remained localised until entering a period of rapid northward range expansion in the 1970s. 3. The present study tested the survival of overwintering F. externa populations from four sites in the north and south of the invaded range (n = 8 sites total) when exposed to -15°C for 0-36 h. 4. This experiment was repeated on F. externa offspring that had been reared in a common-garden environment in order to control for parental effects. 5. Northern populations were more tolerant of exposure to cold temperatures than were southern populations. This held true in both the source-population experiment and common-garden experiment. The common-garden experiment demonstrates that this difference has a genetic basis and may be the consequence of local adaptation to lower winter temperatures. 6. The results provide evidence for local adaptation to extreme temperatures in F. externa. This provides one possible explanation for the lag period between the arrival of this species and its eventual northward range expansion. © 2008 The Authors.

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Ecological Entomology