To reveal the genetic diversity and to infer potential source areas of adventive western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), in Shandong, China, we used mitochondrial and microsatellite markers to analyze the genetic diversity of 15 populations from Shandong, as well as 3 populations from Yunnan and 2 populations from Beijing—these latter 2 sites having the earliest populations to establish in China—and 2 populations from California, which are part of the pest's native range in North America. Data involving the mtCOI gene and microsatellite markers showed that the Chinese populations were less diverse genetically than the native USA populations. The distribution of mtCOI haplotypes and percentage of shared alleles in this study suggested that the populations from Shandong may have arrived as a secondary incursion from Yunnan. We found that the diversity of mitochondrial alleles in some populations from Shandong had declined drastically, whereas the diversity of their nuclear alleles had remained high, i.e., the drastic loss of mitochondrial haplotype diversity in some populations was not accompanied by substantial reductions in nuclear allelic diversity. Therefore, further analyses of nuclear genetic diversity may demonstrate that it provides a better indication of the adaptability of an adventive species than mitochondrial genetic diversity. Also, the F_ST data and genetic diversity analysis suggest that the substantial gene flow among the Shandong populations might have minimized the bottleneck effects.