Monitoring of Treehopper (Hemiptera: Membracidae) Populations in Northern California Vineyards Abstract Two studies were conducted in a project aimed at increasing knowledge of potential treehopper (Hemiptera: Membracidae) vectors of grapevine red blotch virus (family: Geminiviridae) in Northern California vineyards. In the first study, I collected insects identified as Tortistilus species from purple vetch plants in a California vineyard representing individuals with either brown or green coloration and with prominent pronotal suprahumeral horns or without horns. The horned insects were initially thought to be Tortistilus albidosparsus (Stål 1860) and the unhorned insects Tortistilus wickhami (Van Duzee 1908) and the specimens collected appeared in both green and brown color forms. However, their occurrence and feeding on the same host on the same date seemed curious and raised concern about their classification. To ascertain their taxonomic status precisely, two insects of each morphotype: brown horned, brown unhorned, green horned and green unhorned, were subjected to shotgun DNA sequencing using Illumina Hiseq4000. Initial analysis on the assembled contigs of cytochrome oxidase I (COI) indicated that all eight insects were in the same phylogenetic group and had <2% difference with T. wickhami sequences in GenBank. There were no reference sequences for Tortistilus albidosparsus in Genbank. Examination of genitalia of the males did not reveal any morphological differences among the four morphotypes. Matings of combinations of horned and unhorned individuals resulted in F1 adults of both morphotypes. The second study involved monitoring the three-cornered alfalfa hopper, Spissistilus festinus (Say), (Hemiptera: Membracidae) in California vineyards for two years during which the timing and within-plant distribution of its feeding on grapevines was characterized. Spissistilus festinus feeding results in a distinctive ring around the entire stem and petiole, referred to as a girdle. Thirty grapevines from two vineyards in Napa and Solano Counties were thoroughly searched for the presence of girdles every two weeks following budbreak and continuing through leaf fall. Once girdles were counted, each girdled stem and petiole was removed to prevent them from being counted again later. Results suggest that S. festinus begins to feed on grapevines in late May through June, and that feeding on petioles relative to stems differs throughout the year. This information will be useful for Integrated Pest Management decision-making by indicating the best times of the year to apply control measures to prevent feeding damage and help stop the spread of grapevine red-blotch virus by this species.