Abstract:
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.
