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Simple Summary: Globalization accelerates the mixing of populations, including those of humans, animals and plants. Consequently, some species invade new natural communities and alter them. One illustrative example is the spongy moth, which was accidentally introduced into North America, where it became a major forest pest after its establishment there. In its native Eurasian environment, the spongy moth's range has recently shown significant northward expansion, presumably as a consequence of global warming. In its North American range, a similar northward advance is yet to be fully documented. Eurasian populations of the spongy moth show significant developmental plasticity, which allows them to establish themselves in the much colder northern regions due to an acceleration of larval development. The mechanism responsible for this acceleration is not well understood, but our earlier studies point to genetic and epigenetic factors as playing a role. This plasticity indicates that new introductions of Eurasian spongy moths from the northernmost populations pose a more significant threat to North America than previously expected. The spongy moth, Lymatria dispar, is a classic example of an invasive pest accidentally introduced from Europe to North America, where it has become one of the most serious forest defoliators, as in its native range. The present study was aimed at (i) identifying the current northern limit of L. dispar's Eurasian range and exploring its northward expansion in Canada using pheromone trap data, and (ii) comparing northern Eurasian populations with those from central and southern regions with respect to male flight phenology, the sums of effective temperatures (SETs) above the 7 °C threshold necessary for development to the adult stage, and heat availability. We show that the range of L. dispar in Eurasia now reaches the 61st parallel, and comparisons with historical data identify the average speed of spread as 50 km/year. We also document the northern progression of L. dispar in southern Canada, where the actual northern boundary of its range remains to be identified. We show that the median date of male flight does not vary greatly between northern and southern regions of the spongy moth range in Eurasia despite climate differences. Synchronization of flight at different latitudes of the range is associated with an acceleration of larval development in northern Eurasian populations. Similar changes in developmental rate along a latitudinal gradient have not been documented for North American populations. Thus, we argue that this feature of spongy moths from northern Eurasia poses a significant invasive threat to North America in terms of enhanced risks for rapid northward range expansion. [ABSTRACT FROM AUTHOR]