Surveying for potential grapevine red blotch virus vectors in an Oregon vineyard

Grapevine red blotch virus (GRBV) reduces grapevine vigor, delays grape ripening, reduces sugar content and quality of wines in North America and other continents. To date, the three-cornered alfalfa hopper, Spissistilus festinus, has been clearly identified as a vector in the United States. Since this hopper has not been observed in some vineyards where secondary spread occurs, additional vectors of GRBV are likely. To search for vectors, we conducted a two-year survey collecting live hemipterans from infected vines and surrounding grass, trees, and shrubs. Screening a wide variety of hemipterans from a vineyard with infected plants focuses on relevant insects and avoids the difficulty of rearing multiple species. Most collected insects were immediately used in greenhouse transmission tests to infect clean grapevines, and a subset of insects were frozen for PCR tests. Out of 147 transmission tests using 245 plants, four plants were infected after exposure to the leafhoppers Euscelidius variegatus or Dikraneura absenta or delphacid Nothodelphax sp. Whole-body PCR tests of both E. variegatus and D. absenta frozen directly from the field were positive confirming that they ingested GRBV from the field (no Nothodelphax tested). PCR tests of the cixiid Cixius sp. and leafhoppers Fieberiella florii and Gyponana octolineata were positive but the grapevine transmission tests did not support these as vectors. This survey has identified three hemipterans for further transmission studies and for specific GRBV tests of salivary glands to fully confirm vectoring potential.