Graphocephala atropunctata (Signoret) is the principal vector of Xylella fastidiosa (Wells, Raju, Hung, Weisberg, Mandelco-Paul and Brenner), the bacterium that causes Pierce's disease of grapevine in coastal California. Monitoring the activity of G. atropunctata in the early spring is important for timing insecticide sprays and assessing the potential for disease spread to adjacent vineyards. Trapping studies with yellow sticky traps over 3 yr in Napa Valley, CA, established a significant correlation between early spring trap catch and temperature. Sticky trap catches of G. atropunctata occurred in the springs of 1996-1998 only when temperature was greater than or equal to 14.5 degrees C. In 1997 and 1998, the degree-hours (>14.5 degrees C) per day from sunrise to sunset during March and April, but not in May, correlated significantly with trap catches. The temperature threshold of 14.5 degrees C in the early spring can be used to improve the timing of insecticidal applications aimed at reducing G. atropunctata populations in vineyards affected by Pierce's disease.