The leaf-curl plum aphid, Brachycaudus helichrysi (Kaltenbach 1843), is a polyphagous cosmopolitan aphid. This serious pest of plum and peach orchards affects the growth of its host plants by curling infested leaves, and can transmit viruses such as the plum pox virus. Phylogenetic analyses have recently shown that B. helichrysi actually comprises two lineages that are morphologically similar but genetically different. In this study, we developed a PCR-RFLP test for differentiating between these two lineages, based on a mitochondrial DNA fragment. We used this test to investigate the patterns of host plant association and the geographical distribution of these lineages, with a large sample of individuals collected from numerous host plants and locations worldwide. We found that one of the lineages was never present on plum trees, and that individuals from both lineages coexisted on many secondary herbaceous hosts at almost all of the sites studied. We then carried out principal component analysis to determine whether the distribution of the two lineages could be explained by climatic features. We found that the lineage absent from plum trees was not found in regions with harsh winters. This suggests that one of the lineages within B. helichrysi may be an obligate parthenogenetic aphid.