Several mealybug and two soft scale species have been identified as vectors of Grapevine leafroll-associated virus 3 (GLRaV-3), the most abundant of the Closteroviridae associated with grapevine leafroll disease. To identify further soft scale vectors of GLRaV-3, the ability of three species, Coccus longulus, Parasaissetia nigra and a Saissetia sp., to transmit the virus to grapevine was determined under laboratory conditions. Using cultured soft scales, first-instar nymphs of C. longulus and P. nigra were given acquisition access to GLRaV-3-infected rootstock hybrid LN33. Saissetia sp. was reared on LN33 infected with GLRaV-3 and Grapevine virus A (GVA). Nymphs were transferred from virus source to virus-free grapevine plants (cv. Cabernet franc). Recipient plants were tested for GLRaV-3 and GVA with nested reverse transcription polymerase chain reaction (nested RT-PCR) and RT-PCR, respectively. The study shows for the first time that C. longulus, P. nigra and a Saissetia sp. are vectors of GLRaV-3. Saissetia sp. did not transmit GVA. The biology of C. longulus and P. nigra on grapevine was examined at different constant temperatures ranging between 18 and 35 °C, and at 25 and 30 °C, respectively. None of the nymphs survived past the second-instar stage except for one C. longulus female at 30 °C, which produced 117 offspring. The low survival rate could explain the low abundance and patchy distribution of soft scales in South African vineyards. However, outbreaks of soft scales in European vineyards have been reported and this study shows that more soft scale insect species than hitherto thought are able to transmit the virus.
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Vol. 21 • No. 1