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The host range of a given parasitoid species is shaped by host acceptance and suitability traits of the host and parasitoid species, which can themselves be influenced by the parasitoid genotype. However, during biological control introductions, relatively few parasitoid individuals are introduced into a new geographic region and the resulting founder event may alter their ability to attack and parasitize various host species. The aphid parasitoid Lysiphlebus testaceipes is native to the Nearctic region and was introduced from Cuba to Southern Europe in the early 1970s to control two aphid pests of Citrus. We compared parasitoid preference and performance between members of introduced (French) and native (North American) populations of L. testaceipes on 16 aphid species and found differences in various aspects of host specificity. Individuals of the French population exhibited lower virulence overall than individuals of the North American population did. In particular, French L. testaceipes exhibited a lower capability to complete development within five aphid species than North American L. testaceipes did. F1 offspring of crosses between the two parasitoid populations demonstrated significant improvements in virulence on an aphid species on which members of the French parental population exhibited poor virulence. This suggests a relatively simple genetic basis of these performance traits in L. testaceipes. [ABSTRACT FROM AUTHOR]