A new pest species of Ferrisia (Hemiptera: Pseudococcidae), Ferrisia gilli Gullan, is described and illustrated based on the adult female, third-instar female, and first- and second-instar nymphs. Observation of pharate third-instar males and females failed to reveal sexual dimorphism of the second instar. Diagnosis of this new species was facilitated by the collection of nucleotide sequence data from fragments of a mitochondrial gene (COI) and two nuclear genes (EF-1α and 28S). The first phylogenetic study of Ferrisia is presented; combined and separate analyses of the three gene regions support monophyly of F. gilli and suggest that Ferrisia virgata (Cockerell) is a species complex. The known distribution of F. gilli from California and the southeastern United States is reported. It may be native to the southeastern states. This mealybug seems to be polyphagous because it feeds on a variety of species of woody plants, both evergreen and deciduous, as well as on monocots. It has at least three generations annually in central California, where it is newly recognized as a pest in pistachio and almond orchards, but has been present in northern California since at least 1968. The main problems caused by this mealybug in pistachio orchards are contamination of foliage and fruit with honeydew and the concomitant promotion of two major fungal pathogens.