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Abstract: Many researchers have hypothesized that reduced genetic diversity in introduced populations of biological control agents can compromise the agent’s ability to establish and then adapt to novel environments. Strategies such as collecting from a wide geographic area or maintaining large colony sizes through the quarantine process have been used to minimize the loss of genetic diversity. Few studies have examined how the process of collection, quarantine, and establishment could affect patterns of genetic diversity in biological control agents. In this paper, we examine patterns in neutral genetic diversity in Spurgia capitigena, a gall midge introduced as a biological control agent of leafy spurge (Euphorbia esula) from its original collection site in Italy, a newly discovered population of the midge from southern France, and in two established populations in North America. By comparing the existing populations with historical samples collected shortly after the original introductions in 1991, we found that there is evidence of a mild bottleneck in both of the introduced populations. Further, we also examined the population structure in European populations of the fly collected from leafy spurge and a closely related species, cypress spurge (Euphorbia cyparissias) and found evidence for local restrictions in gene flow between populations on the two plants, but no evidence to support the current taxonomy of the genus with two distinct fly species. [Copyright &y& Elsevier]