Herbicides against stinging nettle to control grapevine bois noir disease:does the timing of application affect the emergence of its vector Hyalesthes obsoletus?

Abstract: The planthopper Hyalesthes obsoletus is the principal vector of the grapevine yellowsdisease ‘bois noir’ in Swiss vineyards and stinging nettle (Urtica dioica) is its favourite hostplant. Viticultural control practices therefore target stinging nettle, the actual reservoir and sourceof both the disease and the vector. In order to kill developing nymphs of H. obsoletus, it iscurrently recommended to apply herbicides against stinging nettle in the end of the season. Totest if this late period of herbicide application is justified, stinging nettles were treated withglyphosate in the fall, in the spring or not at all and emergence traps were placed in the centre oftreated and untreated patches for studying herbicide’s direct impact on the vector. Although theautumnal treatment was slightly more efficient, herbicide applications at both dates controlled thegrowth of stinging nettle in the subsequent summer very well. However, emerging H. obsoletusadults were captured in all three treatments and there was no significant difference among them.The aerial application of glyphosate did therefore not restrain nymphs’ development in the soil.Government agencies should consequently reconsider if the authorisation of herbicideapplications against potential host plants in autumn is justified since our results suggest thatstinging nettle could also be controlled in spring, alike other viticultural weeds.