The Cactus Moth, Cactoblastis cactorum, native to southern South America has long been known to be one of the most effective biological control agents of invasive plants (in this case Prickly Pear Cactus, Opuntia sp.) in various parts of the world. However, C. cactorum was recently (in 1988) discovered Florida, either as the result of an accidental introduction or spread from introduced populations in the Caribbean. It has since spread rapidly through the south-eastern states and is now severely threatening native Opuntia sp. in North America. Efforts to stop this invasion have hitherto been hampered by lack of knowledge on where the original introduction came from, how fast C. cactorum is spreading and whether the already established populations have been boosted by later introductions. Here we present a phylogeographic analysis based on the mitochondrial gene COI of more than 20 C. cactorum populations from south-eastern USA and 10 populations from the Caribbean, South Africa, Australia, New Caledonia and Argentina. The results are used to address the three questions: Where did the original introduction come from? Have there been later introductions? How fast is C. cactorum spreading through the South (and in which directions)? These questions are essential to predicting the expansion of C. cactorum populations and hence employing an effective strategy for stopping the invasion.