Experimental Study on the Dispersal of Grapevine Leafroll‐Associated Viruses ‐1 and ‐3 by the Scale Insects Parthenolecanium corni (Bouché) and Heliococcus bohemicus Šulc in a French Vineyard

ABSTRACT The natural dispersion of scale insect vectors and of grapevine leafroll viruses was investigated in a newly planted virus‐free experimental plot by implanting individual GLRaV‐1 or ‐3 infected vines (‘source vines’) at the centre of sub‐plots. Newborn nymphs of the soft scale Parthenolecanium corni and of the mealybug Heliococcus bohemicus were released on half of these source vines (‘release vines’), inside two separate zones in the plot. Infected vines without insect release were used as controls for virus dispersal. Scale insect dispersal and virus propagation from source vines were monitored during 11 years since the first release of insects in 2013. Our results show that scale insect distribution around source vines extended anisotropically to the next vines along the same row, then to vines of neighbouring rows. Within its initial release zone, P. corni dispersed early from release vines in the first years after introduction, then disappeared progressively. In contrast, a low number of H. bohemicus was recorded up to 2020, then the population showed a wide dispersal from 2021 onwards through almost the whole plot. Moreover, isolated individuals of P. corni and of H. bohemicus were discovered on more distant vines since 2015 and 2016, respectively. In the zone of P. corni release, 18 new vines became infected with GLRaV‐1. Spatial distribution of new GLRaV‐1‐infected vines showed that contamination progressed first from source vines to the next neighbouring vines on the same row, then to opposite vines on the next rows. In the H. bohemicus zone, despite its late and wide dispersal, only one GLRaV‐1‐infected plant was detected, two vines away from a source vine. Difference of virus propagation ability between the two vector species could be due to the low number of H. bohemicus individuals recorded on vines until 2020 and to the low vector capacity of the initial population used for the release.