UC Davis's Foundation Plant Materials Service (FPMS) maintains the disease-tested, professionally identified collection of grape scion and rootstock varieties, which is the core of the California Grapevine Registration and Certification Program. In 1992; newly developed serological testing techniques revealed the presence of grapevine leafroll-associated viruses (GLRaVs) in previously healthy vines in an older foundation propagating block, indicating active and recent virus spread. FPMS responded by increasing isolation distances and implementing a comprehensive virus screening program using the new methodology. The critical problem was the lack of information on leafroll virus epidemiology. When the distribution of infected plants in the old vineyard was mapped, new infections were frequently adjacent to known diseased grapevines. This study examined the ability of mealybugs, a putative leafroll vector, to transmit this group of viruses. We were able to confirm that four species found in California — obscure, longtailed, citrus and grape mealybug — can transmit GLRaV-3 isolates. This is the first experimental evidence of grapevine leafroll virus transmission by obscure and grape mealybug. In addition, we report for the first time that GLRaV-5 can be transmitted by longtailed mealybug.