Transmission of Grapevine Leafroll-associated Virus 3 by Vine Mealybug, Planococcus ficus (Signoret), to Grapevines Treated with Imidacloprid

Grapevine leafroll disease is widely accepted to be the most damaging grapevine virus disease in South
Africa. The current industry strategy to limit leafroll infection in new vineyards includes treating newly
planted grapevines with a systemic insecticide like imidacloprid and rogueing all grapevines that exhibit
leafroll symptoms. A tendency by some producers to eschew rogueing and rely solely on imidacloprid
applications to control vine mealybug and so protect new vineyards from leafroll infection gave rise to this
study. Two bioassays were conducted to determine if Planococcus ficus (Signoret) nymphs can transmit
grapevine leafroll-associated virus 3 (GLRaV-3) to potted indicator grapevines treated with imidacloprid
applied through the soil five months before exposure to viruliferous mealybugs. Ten of 21 leafroll-free
grapevines treated with imidacloprid tested positive for GLRaV-3 seven to eight weeks after being exposed
to viruliferous first- and second-instar P. ficus nymphs. This means that the systemic applications of
imidacloprid to newly planted, virus-free vineyards will not necessarily protect the vines from GLRaV-3 if
they become infested by viruliferous vine mealybugs, although the insecticide provides effective mealybug
control and helps to prevent the secondary spread of leafroll infection in these vineyards. To control
the spread of leafroll virus to newly planted, leafroll-free vineyards, all grapevines that serve as sources
of infection should be removed. Where this is not feasible, vectors should be monitored and controlled
rigorously in infested and disease-free vineyards alike, while all infected grapevines should be removed
from new vineyards as they begin to develop leafroll symptoms.