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Temperature provides important physiological constraints that can influence the distribution of an invasive species. Gypsy moth (Lymantria disparL.) is a generalist defoliator in North America and supraoptimal temperatures (above the optimal for developmental rate) have been implicated in range dynamics at the southern invasion front in West Virginia and Virginia. We sourced egg masses from the Appalachian Mountains (AM), where the gypsy moth range is expanding, from the Coastal Plain (CP), where range retraction is occurring, and from a long-established population in New York (NY) and conducted a reciprocal transplant experiment to compare development and fitness components among these populations at two sites along the southern invasion front. We found evidence of sublethal effects from rearing in the CP, with decreased pupal mass and fewer eggs compared to individuals reared in the AM, but little difference between source populations in developmental traits. The AM and NY populations did experience reductions in egg viability under a southern winter at the CP site compared to control wintering conditions, while the CP egg masses had equivalent survival. This study provides empirical support for negative fitness consequences of supraoptimal temperatures at the southern range edge, consistent with patterns of range retraction and spread in the region, as well as suggesting the potential for local adaptation through variation in egg survival. Our work illustrates that sublethal effects from high temperature can be an important factor determining the distribution of invasive species under current and future climates.