The western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), is an invasive species and currently occurs in only a few areas in China. An easy, accurate and developmental-stage independent method to identify F. occidentalis would be a valuable tool to facilitate pest management decision making and, more importantly, to provide an early warning so actions can be taken to prevent its introduction into non-infested areas. Morphological identification of thrips adults and, to a lesser extent, of second-stage larvae is the main method currently available to identify F. occidentalis. Molecular identification, however, can be easily carried out by a non-thrips-specialist with a little training. In this study, DNA sequence data [within the mitochondrial cytochrome oxidase I gene ( COI)] and polymerase chain reaction (PCR) were utilized to develop a molecular diagnostic marker for F. occidentalis. A primer set and PCR cycling parameters were designed for the amplification of a single marker fragment (340 bp) of F. occidentalis COI mtDNA. Specificity tests performed on 28 thrips species, efficacy tests performed on five immature developmental stages as well as on male and female adults and tests on primer sensitivity all demonstrated the diagnostic utility of this marker. Furthermore, the primer set was tested on seventeen F. occidentalis populations from different countries and invaded areas in China and proved to be applicable for all geographic populations. It was used successfully to clarify the distribution of F. occidentalis in the Beijing metro area. These results suggested that this diagnostic PCR assay provides a quick, simple and reliable molecular technique for the identification of F. occidentalis. [ABSTRACT FROM AUTHOR]