AbstractThe greenhouse whiteflyTrialeurodes vaporariorumWestwood, 1856 (Hemiptera: Aleyrodidae) is an invasive and highly polyphagous phloem-feeding pest of vegetables and ornamentals.Trialeurodes vaporariorumcauses serious damage due to direct feeding and transmits several important plant viruses. Excessive use of insecticides has resulted in significantly reduced levels of susceptibility of variousT. vaporariorumpopulations. To determine the genetic variability within and among populations ofT. vaporariorumfrom Serbia and to explore their genetic relatedness with otherT. vaporariorumpopulations, we analysed the mitochondrial cytochrome c oxidase I (COI) sequences of 16 populations from Serbia and six neighbouring countries: Montenegro (three populations), Macedonia (one population) and Croatia (two populations), for a total of 198 analysed specimens. A low overall level of sequence divergence and only five variable nucleotides and six haplotypes were found. The most frequent haplotype, H1, was identified in all Serbian populations and in all specimens from distant localities in Croatia and Macedonia. The COI sequence data that was retrieved from GenBank and the data from our study indicated that H1 is the most globally widespreadT. vaporariorumhaplotype. A lack of spatial genetic structure among the studiedT. vaporariorumpopulations, as well as two demographic tests that we performed (Tajima'sDvalue and Fu's Fs statistics), indicate a recent colonisation event and population growth. Phylogenetic analyses of the COI haplotypes in this study and otherT. vaporariorumhaplotypes that were retrieved from GenBank were performed using Bayesian inference and median-joining (MJ) network analysis. Two major haplogroups with only a single unique nucleotide difference were found: haplogroup 1 (containing the five Serbian haplotypes and those previously identified in India, China, the Netherlands, the United Kingdom, Morocco, Reunion and the USA) and haplogroup 3 (containing the single Serbian haplotype H3 and haplotypes from Costa Rica, the USA and Spanish Canary Islands). Collectively, our data indicate a rather limited value of COI as a genetic marker for discrimination between differentT. vaporariorumpopulations in the investigated area. Possible explanations for the observed lack of COI sequence variability, such as specific genetics of biological invasion and/or the influence of bacterial symbionts that manipulate insect reproduction, are discussed.