Transmission efficiency of Grapevine leafroll-associated virus 3 (GLRaV-3) by the mealybugs Planococcus ficus and Pseudococcus longispinus (Hemiptera: Pseudococcidae)

Grapevine leafroll-associated virus 3 (GLRaV-3) is transmitted from grapevine to grapevine through grafting infected material, as well as mealybug and soft scale insect vectors. We determined the transmission efficiency of GLRaV-3 by two mealybug vectors, Planococcus ficus and Pseudococcus longispinus, by examining the relationship between the number of nymphs per plant, using groups of 1, 5, 10, 20 and 40 first- to second-instar nymphs, and the infection rate of healthy grapevine plants. Plants and single nymphs were tested for GLRaV-3 with nested reverse transcription polymerase chain reaction (nested RT-PCR). The results of this study showed the infection rate of P. ficus and P. longispinus nymphs with GLRaV-3 after acquisition access and inoculation access periods of 5 days each to be similar between intra- and interspecific groups, ranging from 5% to 19%, except for the '40-nymph group' of P. longispinus, which had a higher number of infected nymphs (63%). The number of nymphs used per group did not influence the infection rate of plants, which ranged from 11% to 80% and 20% to 70% for P. ficus and P. longispinus, respectively. Transmission of GLRaV-3 with single P. ficus and P. longispinus nymphs resulted in 70% GLRaV-3 positive plants in both cases. This study has shown both species to be equally efficient as vectors of GLRaV-3. We have further shown for the first time that a single individual of either P. ficus or P. longispinus is capable of transmitting GLRaV-3 and infecting a healthy grapevine plant. The results of the study highlight the importance of implementing vector control measures.